Handover Method and Apparatus

ABSTRACT

A handover method and apparatus, where the method includes: determining, by a first control plane function network element, at least one bearer that needs to be established when a protocol data unit (PDU) session of a terminal in a first network is to be switched to a packet data network (PDN) connection in a second network; and obtaining, by the first control plane function network element, tunnel information of the PDN connection, where the tunnel information of the PDN connection includes user plane tunnel information corresponding to each of the at least one bearer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/992,880, filed on Aug. 13, 2020, which is a continuation ofInternational Patent Application No. PCT/CN2019/074825, filed on Feb.12, 2019, which claims priority to Chinese Patent Application No.201810152399.3, filed on Feb. 14, 2018. All of the aforementioned patentapplications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

This application relates to the communications field, and morespecifically, to a handover method and apparatus in the communicationsfield.

BACKGROUND

With rapid development of wireless communications technologies, a fifthgeneration (5G) mobile communications technology emerges. In an initialstage of network deployment, due to insufficient coverage of a 5Gnetwork, when a location of a user equipment (UE) changes, the UE needsto be handed over between the 5G network and a fourth generation (4G)network.

In the 4G network, one packet data network (PDN) connection correspondsto one or more bearers. Each bearer includes one or more data flows. Asystem establishes a corresponding tunnel for each bearer correspondingto the PDN connection. In a handover preparation process, a sourcemobility management entity (MME) sends tunnel information of each bearercorresponding to the PDN connection to a target MME, and establishes,for each bearer, an uplink tunnel from a target access network to apacket data network (PDN) gateway user plane function (PGW-U).

One protocol data unit (PDU) session in the 5G network is equivalent toone PDN connection in the 4G network. One PDU session includes one ormore data flows, and all the data flows in the PDU session share onetunnel.

In the 5G network, one tunnel is used for one PDU session, whereas inthe 4G network, one tunnel is required for each bearer of one PDNconnection. Therefore, when the UE is handed over from the 5G network tothe 4G network, there is a problem of how to map one tunnel to aplurality of tunnels. Similarly, when the UE is handed over from the 4Gnetwork to the 5G network, there is a problem of how to map a pluralityof tunnels to one tunnel.

Therefore, when the UE is handed over between the 4G network and the 5Gnetwork, data may fail to be normally received and processed, andconsequently, the data is lost.

SUMMARY

This application provides a handover method and apparatus, such that adata loss in a network handover process can be prevented, therebyensuring normal data transmission.

According to a first aspect, this application provides a handovermethod. The method includes: determining, by a first control planefunction network element, at least one bearer that needs to beestablished when a protocol data unit (PDU) session of a terminal in afirst network is to be switched to a packet data network (PDN)connection in a second network; and obtaining, by the first controlplane function network element, tunnel information of the PDNconnection, where the tunnel information of the PDN connection includesuser plane tunnel information corresponding to each of the at least onebearer.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element determinesthe at least one bearer that needs to be established when the PDUsession of the terminal in the first network is to be switched to thePDN connection in the second network, and obtains the user plane tunnelinformation corresponding to each of the at least one bearer. As such,the first control plane function network element establishes a userplane tunnel corresponding to a bearer in the PDN connection when thePDU session is to be switched to the PDN connection. This helps preventa loss of a data packet in the bearer included in the PDN connection ina network handover process, thereby ensuring normal transmission of thedata packet.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. It should befurther understood that, the PDU session in the first network isequivalent to the PDN connection in the second network.

It should be understood that, the first control plane function networkelement may be, for example, a session management function (SMF) pluspacket data network gateway (PGW) control plane (SMF-PGW-C).

Optionally, the user plane tunnel information corresponding to eachbearer may include a tunnel identifier of a user plane tunnelcorresponding to the bearer and other information about the user planetunnel, such as an Internet Protocol (IP) address corresponding to thetunnel. This is not limited in this embodiment of this application.

Optionally, the first control plane function network element maydetermine, in a plurality of cases, that the PDU session in the firstnetwork needs to be established or modified. This is not limited in thisembodiment of this application.

In a possible implementation, the first control plane function networkelement may receive a first request message from an access and mobilitymanagement function (AMF), where the first request message is used torequest to establish the PDU session in the first network, and the PDUsession includes at least one data flow. Additionally, the first controlplane function network element determines, based on the first requestmessage, that the PDU session in the first network needs to beestablished.

In another possible implementation, the first control plane functionnetwork element may receive a second request message from anothernetwork element, where the second request message is used to trigger thefirst control plane function network element to modify the PDU sessionin the first network. For example, the second request message is used totrigger the first control plane function network element to add at leastone data flow to the PDU session.

In still another possible implementation, the first control planefunction network element may be triggered, by a process of the firstcontrol plane function network element, to modify the PDU session in thefirst network.

Optionally, the first control plane function network element maydetermine the at least one bearer based on the at least one data flowand an allocation policy.

Optionally, the at least one data flow may be, for example, a quality ofservice (QoS) flow. A QoS flow may be a guaranteed bit rate (GBR) QoSflow or a non-GBR QoS flow.

Optionally, the allocation policy may be pre-configured, or may beindicated by another network element using indication information. Thisis not limited in this embodiment of this application.

With reference to the first aspect, in some possible implementations,the obtaining, by the first control plane function network element,tunnel information of the PDN connection, where the tunnel informationof the PDN connection includes user plane tunnel informationcorresponding to each of the at least one bearer includes: allocating,by the first control plane function network element, the correspondinguser plane tunnel information to each of the at least one bearer; orrequesting, by the first control plane function network element, a userplane function network element corresponding to the PDU session toallocate user plane tunnel information corresponding to each bearer, andobtaining the user plane tunnel information corresponding to each bearerfrom the user plane function network element corresponding to the PDUsession.

Optionally, the first control plane function network element may obtain,in a plurality of manners, the user plane tunnel informationcorresponding to each of the at least one bearer. This is not limited inthis embodiment of this application.

In a possible implementation, the first control plane function networkelement itself may allocate the user plane tunnel information to each ofthe at least one bearer.

In another possible implementation, the first control plane functionnetwork element may request the user plane function network elementcorresponding to the PDU session to allocate user plane tunnelinformation to each of the at least one bearer, and obtain the userplane tunnel information corresponding to each bearer from the userplane function network element corresponding to the PDU session.

With reference to the first aspect, in some possible implementations,the tunnel information of the PDN connection further includes controlplane tunnel information corresponding to the PDN connection.Additionally, the obtaining, by the first control plane function networkelement, tunnel information of the PDN connection further includesobtaining, by the first control plane function network element, thecontrol plane tunnel information corresponding to the PDN connection.

Optionally, the control plane tunnel information corresponding to thePDN connection may include a tunnel identifier of a control plane tunnelcorresponding to the PDN connection and other information about thecontrol plane tunnel, such as an IP address corresponding to the controlplane tunnel. This is not limited in this embodiment of thisapplication.

According to the handover method provided in this embodiment of thisapplication, the tunnel information corresponding to the PDN connectionfurther includes the control plane tunnel information corresponding tothe PDN connection, such that the first control plane function networkelement establishes the control plane tunnel corresponding to the PDNconnection corresponding to the PDU session. This helps prevent a lossof control signaling of the PDN connection in the network handoverprocess, thereby ensuring normal transmission of the control signaling.

With reference to the first aspect, in some possible implementations,the method further includes sending, by the first control plane functionnetwork element, a first session management request to the user planefunction network element corresponding to the PDU session. The firstsession management request is used to request to establish or modify auser plane tunnel corresponding to each of the at least one bearer, thefirst session management request carries an uplink data transmissionrule, and the uplink data transmission rule is used to indicate the userplane function network element corresponding to the PDU session how toforward uplink data that is received through the user plane tunnelcorresponding to the PDN connection.

Optionally, when the first control plane network element itselfallocates the user plane tunnel information to each of the at least onebearer, the first control plane function network element may add theuser plane tunnel information corresponding to each of the at least onebearer to the first session management request. Additionally, when thefirst control plane function network element requests the user planefunction network element corresponding to the PDU session to allocateuser plane tunnel information to each of the at least one bearer, theuser plane function network element corresponding to the PDU session mayadd user plane tunnel information corresponding to each of the at leastone bearer to a first session management response.

It should be noted that, after learning of the user plane tunnelinformation corresponding to each of the at least one bearer and anuplink data transmission rule corresponding to the bearer, the userplane function network element corresponding to the PDU session mayestablish an uplink user plane tunnel of the bearer on the user planefunction network element corresponding to the PDU session.

With reference to the first aspect, in some possible implementations,the method further includes obtaining, by the first control planefunction network element, a bearer identifier corresponding to each ofthe at least one bearer.

With reference to the first aspect, in some possible implementations,the obtaining a bearer identifier corresponding to each of the at leastone bearer includes: sending, by the first control plane functionnetwork element, a bearer identifier request to a mobility managementnetwork element using a second control plane function network element,where the bearer identifier request is used to request to allocate abearer identifier to each of the at least one bearer, the second controlplane function network element is a visited control plane networkelement corresponding to the PDU session, and the first control planefunction network element is a home control plane network elementcorresponding to the PDU session; and receiving, by the first controlplane function network element, the bearer identifier corresponding toeach of the at least one bearer from the mobility management networkelement using the second control plane function network element.

In a possible implementation, the first control plane function networkelement may send the bearer identifier request to the mobilitymanagement network element, where the bearer identifier request is usedto allocate a bearer identifier to each of the at least one bearer.Correspondingly, the mobility management network element receives thebearer identifier request from the first control plane function networkelement, allocates a bearer identifier to each bearer based on thebearer identifier request, and sends the bearer identifier correspondingto each of the at least one bearer to the first control plane functionnetwork element.

It should be noted that, the first control plane function networkelement may store, into a bearer context corresponding to the PDNconnection, the user plane tunnel information corresponding to each ofthe at least one bearer, the bearer identifier corresponding to eachbearer, and the control plane tunnel information corresponding to thePDN connection that are obtained, and send the bearer context to themobility management network element during handover preparation.

Correspondingly, the mobility management network element may establish,based on the bearer context corresponding to the PDN connection, anuplink tunnel from a serving gateway (SGW) in the second network to theuser plane function network element corresponding to the PDU session.

In a possible implementation, the first control plane function networkelement sends the bearer identifier request to the mobility managementnetwork element using the second control plane function network element,where the bearer identifier request is used to request to allocate abearer identifier to each of the at least one bearer. Additionally, thefirst control plane function network element receives the beareridentifier corresponding to each of the at least one bearer from themobility management network element using the second control planefunction network element.

It should be noted that, after obtaining the user plane tunnelinformation corresponding to each of the at least one bearer, the beareridentifier corresponding to each bearer, and the control plane tunnelinformation corresponding to the PDN connection, the first control planefunction network element may send the user plane tunnel informationcorresponding to each of the at least one bearer, the bearer identifiercorresponding to each bearer, and the control plane tunnel informationcorresponding to the PDN connection to the second control plane functionnetwork element during establishment/modification of the PDU session.

Correspondingly, the second control plane function network element maystore, into the bearer context of the PDN connection, the user planetunnel information corresponding to each of the at least one bearer, thebearer identifier corresponding to each bearer, and the control planetunnel information corresponding to the PDN connection that are obtainedfrom the first control plane function network element.

In another possible implementation, the first control plane functionnetwork element may send the tunnel information of the PDN connection tothe second control plane function network element, and obtain the beareridentifier corresponding to each of the at least one bearer from thesecond control plane function network element. Correspondingly, thesecond control plane function network element may store the beareridentifier corresponding to each of the at least one bearer.

In other words, the first control plane function network element sendsthe tunnel information of the PDN connection to the second control planefunction network element. Additionally, the second control planefunction network element determines, based on the tunnel information ofthe PDN connection, whether the mobility management network elementneeds to be requested to allocate a bearer identifier to each of the atleast one bearer, and sends the bearer identifier request to themobility management network element when the mobility management networkelement needs to be requested to allocate a bearer identifier to each ofthe at least one bearer.

For example, the first control plane function network element firstchecks whether a bearer identifier associated with the user plane tunnelinformation corresponding to each bearer exists in the obtained tunnelinformation of the PDN connection, and if no bearer identifiers areallocated to some of the at least one bearer, the first control planefunction network element requests the mobility management networkelement to allocate bearer identifiers to the bearers.

It should be noted that, the second control plane function networkelement may store, into the bearer context of the PDN connection, theuser plane tunnel information corresponding to each of the at least onebearer, the bearer identifier corresponding to each bearer, and thecontrol plane tunnel information corresponding to the PDN connection.

Because the bearer context corresponding to the PDN connection stored bythe second control plane function network element includes the userplane tunnel information corresponding to each of the at least onebearer, the bearer identifier corresponding to each bearer, and thecontrol plane tunnel information corresponding to the PDN connection,the mobility management network element may obtain the bearer contextcorresponding to the PDN connection from the second control planefunction network element during handover preparation, and establish,based on the bearer context corresponding to the PDN connection, theuplink tunnel from the serving gateway (SGW) in the second network tothe user plane function network element corresponding to the PDUsession.

According to the handover method provided in this embodiment of thisapplication, because the second control plane function network elementis a visited control plane network element corresponding to the PDUsession, compared with obtaining the bearer context corresponding to thePDN connection from the first control plane function network element,obtaining, by the mobility management network element, the bearercontext corresponding to the PDN connection from the second controlplane function network element can reduce a delay generated in thehandover process.

With reference to the first aspect, in some possible implementations,the method further includes sending, by the first control plane functionnetwork element, the tunnel information of the PDN connection and thebearer identifier corresponding to each of the at least one bearer tothe second control plane function network element.

With reference to the first aspect, in some possible implementations,the method further includes: sending, by the first control planefunction network element, the tunnel information of the PDN connectionto a second control plane function network element, where the secondcontrol plane function network element is a visited control planenetwork element corresponding to the PDU session, and the first controlplane function network element is a home control plane network elementcorresponding to the PDU session; and receiving, by the first controlplane function network element, the bearer identifier corresponding toeach of the at least one bearer from the second control plane functionnetwork element.

According to the handover method provided in this embodiment of thisapplication, the bearer context corresponding to the PDN connectionstored by the second control plane function network element includes thetunnel information of the PDN connection and the bearer identifiercorresponding to each of the at least one bearer, such that the mobilitymanagement network element may directly obtain the bearer contextcorresponding to the PDN connection from the second control planefunction network element during handover preparation, and establish,based on the bearer context corresponding to the PDN connection, theuplink tunnel from the SGW in the second network to the user planefunction network element corresponding to the PDU.

With reference to the first aspect, in some possible implementations,the obtaining, by the first control plane function network element,tunnel information of the PDN connection includes obtaining, by thefirst control plane function network element, the tunnel information ofthe PDN connection when determining that the PDU session of the terminalneeds to be established in the first network or the PDU session in thefirst network needs to be modified.

Optionally, the first control plane function network element may obtainthe tunnel information of the PDN connection during sessionestablishment/modification. Alternatively, the first control planefunction network element may obtain the tunnel information of the PDNconnection during handover preparation.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element obtainsthe tunnel information of the PDN connection during establishment ormodification of the PDU session or during handover preparation. As such,the first control plane function network element establishes, beforehandover execution, the user plane tunnel corresponding to the bearer inthe PDN connection and the control plane tunnel corresponding to the PDNconnection, thereby reducing a delay in the handover process.

With reference to the first aspect, in some possible implementations,the method further includes sending, by the first control plane functionnetwork element to the user plane function network element correspondingto the PDU session, a downlink data transmission rule corresponding tothe PDN connection, where the downlink data transmission rule is used toindicate the user plane function network element corresponding to thePDU session to forward received downlink data through a correspondinguser plane tunnel.

Optionally, the first control plane function network element may send,to the user plane function network element corresponding to the PDUsession, the downlink data transmission rule corresponding to the PDNconnection, where the downlink data transmission rule is used toindicate the user plane function network element corresponding to thePDU session to forward received downlink data through a correspondinguser plane tunnel.

In a possible implementation, the first control plane function networkelement may send a third session management request to the user planefunction network element corresponding to the PDU session duringhandover execution, where the third session management request is usedto request to establish or modify a downlink user plane tunnelcorresponding to each of the at least one bearer, and the third sessionmanagement request carries the downlink data transmission rule.

In another possible implementation, the first control plane functionnetwork element may add the downlink data transmission rule and firstrule indication information to the first session management requestduring PDU session request/modification, where the first rule indicationinformation is used to indicate not to enable the downlink datatransmission rule. Correspondingly, the first control plane functionnetwork element sends a third session management request to the userplane function network element corresponding to the PDU session duringhandover execution, where the third session management request carriessecond rule indication information, and the second rule indicationinformation is used to indicate to enable the downlink data transmissionrule.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element indicatesthe downlink data transmission rule to the user plane function networkelement corresponding to the PDU session in a handover execution phase.As such, the user plane function network element corresponding to thePDU session matches received downlink data to a corresponding downlinktunnel according to the downlink data transmission rule, and sends thereceived downlink data to the serving gateway in the second networkthrough the downlink tunnel.

According to a second aspect, this application provides a handovermethod. The method includes: receiving, by a second control planefunction network element from a first control plane function networkelement, tunnel information of a PDN connection from a first controlplane function network element, where the tunnel information of the PDNconnection includes user plane tunnel information corresponding to eachof at least one bearer, the second control plane function networkelement is a visited control plane network element corresponding to thePDN connection, and the first control plane function network element isa home control plane network element corresponding to the PDNconnection; and sending, by the second control plane function networkelement, the tunnel information of the PDN connection to a mobilitymanagement network element.

According to the handover method provided in this embodiment of thisapplication, a bearer context corresponding to the PDN connection storedby the second control plane function network element includes the tunnelinformation of the PDN connection and a bearer identifier correspondingto each of the at least one bearer. As such, the mobility managementnetwork element may directly obtain the bearer context corresponding tothe PDN connection from the second control plane function networkelement during handover preparation, and establish, based on the bearercontext corresponding to the PDN connection, an uplink tunnel from aserving gateway (SGW) in the second network to a user plane functionnetwork element corresponding to the PDU.

In addition, because the second control plane function network elementis a visited control plane network element corresponding to the PDUsession, compared with obtaining the bearer context corresponding to thePDN connection from the first control plane function network element,obtaining, by the mobility management network element, the bearercontext corresponding to the PDN connection from the second controlplane function network element can reduce a delay generated in ahandover process.

It should be understood that, the first control plane function networkelement may be, for example, an SMF-PGW-C, and the second control planefunction network element may be, for example, a v-SMF.

With reference to the second aspect, in some possible implementations,the tunnel information of the PDN connection further includes controlplane tunnel information corresponding to the PDN connection.

According to the handover method provided in this embodiment of thisapplication, the tunnel information corresponding to the PDN connectionfurther includes the control plane tunnel information corresponding tothe PDN connection, such that the first control plane function networkelement establishes a control plane tunnel corresponding to the PDNconnection corresponding to the PDU session. This helps prevent a lossof control signaling of the PDN connection in a network handoverprocess, thereby ensuring normal transmission of the control signaling.

With reference to the second aspect, in some possible implementations,the method further includes: forwarding, by the second control planefunction network element to the mobility management network element, abearer identifier request that is sent by the first control planefunction network element to the mobility management network element,where the bearer identifier request is used to request to allocate abearer identifier to each of the at least one bearer; and forwarding, bythe second control plane function network element to the first controlplane function network element, the bearer identifier that correspondsto each of the at least one bearer and that is sent by the mobilitymanagement network element to the first control plane function networkelement.

In other words, the second control plane function network element has afunction of transparently transmitting a message transmitted between thefirst control plane function network element and the user plane functionnetwork element corresponding to the PDU session.

With reference to the second aspect, in some possible implementations,the method further includes receiving, by the second control planefunction network element, the bearer identifier corresponding to each ofthe at least one bearer from the first control plane function networkelement.

With reference to the second aspect, in some possible implementations,the method further includes: sending, by the second control planefunction network element, a bearer identifier request to the mobilitymanagement network element based on the tunnel information of the PDNconnection, where the bearer identifier request is used to request toallocate a bearer identifier to each of the at least one bearer; andreceiving, by the second control plane function network element, thebearer identifier corresponding to each of the at least one bearer fromthe mobility management network element.

For example, the second control plane network element may check whethera bearer identifier associated with the user plane tunnel informationcorresponding to each bearer exists in the tunnel information of the PDNconnection, and if no bearer identifier is allocated to the at least onebearer, the second control plane function network element requests themobility management network element to allocate the bearer identifier toeach of the at least one bearer to which no bearer identifier isallocated.

In other words, the second control plane function network element mayhave a processing function.

With reference to the second aspect, in some possible implementations,the method further includes: storing, by the second control planefunction network element, the bearer identifier corresponding to each ofthe at least one bearer; and sending the bearer identifier correspondingto each of the at least one bearer to the first control plane functionnetwork element.

According to a third aspect, this application provides a handovermethod. The method includes: determining, by a first control planefunction network element, that a PDN connection of a terminal in asecond network needs to be switched to a PDU session in a first network;and obtaining, by the first control plane function network element, userplane tunnel information corresponding to the PDU session.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element determinesthat the PDN connection of the terminal in the second network needs tobe switched to the PDU session in the first network, and obtains theuser plane tunnel information corresponding to the PDU session. As such,the first control plane function network element establishes a userplane tunnel corresponding to the PDU session when the PDN connection isto be switched to the PDU session. This helps prevent a loss of a datapacket in the PDU session in a network handover process, therebyensuring normal transmission of the data packet.

It should be understood that, the first control plane function networkelement may be, for example, an SMF-PGW-C.

It should be further understood that, the PDU session in the firstnetwork is equivalent to the PDN connection in the second network.

In a possible implementation, the first control plane function networkelement may receive a third request message, where the third requestmessage is used to request to establish the PDN connection in the secondnetwork. Additionally, the first control plane function network elementdetermines, based on the third request message, that the PDN connectionof the terminal in the second network needs to be switched to the PDUsession in the first network.

In a possible implementation, the first control plane function networkelement may receive a second session management request, where thesecond session management request carries first indication information,and the first indication information is used to indicate to prepare forhanding over the terminal from the second network to the first network.Additionally, the first control plane function network element maydetermine, based on the first indication information, that the PDNconnection in the second network needs to be switched to the PDU sessionin the first network.

With reference to the third aspect, in some possible implementations,the obtaining, by the first control plane function network element, userplane tunnel information corresponding to the PDU session includes:allocating, by the first control plane function network element to thePDU session, the user plane tunnel information corresponding to the PDUsession; or requesting, by the first control plane function networkelement, a user plane function network element corresponding to the PDNconnection to allocate user plane tunnel information corresponding tothe PDU session, and obtaining the user plane tunnel information thatcorresponds to the PDU session from the user plane function networkelement corresponding to the PDN connection.

Optionally, the first control plane function network element may obtain,in a plurality of manners, the user plane tunnel informationcorresponding to the PDU session. This is not limited in this embodimentof this application.

In a possible implementation, the first control plane function networkelement itself may allocate, to the PDU session, the user plane tunnelinformation corresponding to the PDU session.

In another possible implementation, the first control plane functionnetwork element requests the user plane function network elementcorresponding to the PDN connection to allocate user plane tunnelinformation corresponding to the PDU session, and obtains the user planetunnel information that corresponds to the PDU session from the userplane function network element corresponding to the PDN connection.

With reference to the third aspect, in some possible implementations,the method further includes sending, by the first control plane functionnetwork element, a first session management request to the user planefunction network element corresponding to the PDN connection, where thefirst session management request is used to request to establish ormodify a user plane tunnel corresponding to the PDU session, the firstsession management request carries an uplink data transmission rule, andthe uplink data transmission rule is used to indicate the user planefunction network element corresponding to the PDN connection how toforward uplink data that is received through the user plane tunnelcorresponding to the PDU session.

Optionally, when the first control plane network element itselfallocates the user plane tunnel information to the PDU session, thefirst control plane function network element may add the user planetunnel information corresponding to the PDU session to the first sessionmanagement request. When the first control plane function networkelement requests the user plane function network element correspondingto the PDN connection to allocate user plane tunnel information to thePDU session, the user plane function network element corresponding tothe PDN connection may add the user plane tunnel informationcorresponding to the PDU session to a first session management response.

Optionally, during session modification, the PDN connection in thesecond network may be updated, for example, some bearers are newlyestablished. Therefore, the user plane function network elementcorresponding to the PDN connection needs to know an updated uplink datatransmission rule, in order to associate data flows corresponding tothese bearers with the user plane tunnel corresponding to the PDUsession.

With reference to the third aspect, in some possible implementations,the method further includes: when determining that the uplink datatransmission rule changes, sending, by the first control plane functionnetwork element, an updated uplink data transmission rule to the userplane function network element corresponding to the PDN connection.

Optionally, when the first control plane function network element itselfallocates the user plane tunnel information to the PDU session, thefirst control plane function network element may obtain the user planetunnel information corresponding to the PDU session, and send, to theuser plane function network element corresponding to the PDN connection,the user plane tunnel information corresponding to the PDU session in asame phase; or obtain the user plane tunnel information corresponding tothe PDU session, and send, to the user plane function network elementcorresponding to the PDN connection, the user plane tunnel informationcorresponding to the PDU session in different phases. This is notlimited in this embodiment of this application.

In a possible implementation, the first control plane function networkelement may obtain the user plane tunnel information corresponding tothe PDU session and the uplink data transmission rule during sessionestablishment/modification, and send the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleto the user plane function network element corresponding to the PDNconnection during session establishment/modification.

In another possible implementation, the first control plane functionnetwork element may obtain the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleduring network handover preparation, and send the user plane tunnelinformation corresponding to the PDU session and the uplink datatransmission rule to the user plane function network elementcorresponding to the PDN connection during network handover preparation.

In still another possible implementation, the first control planefunction network element may obtain the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleduring session establishment/modification, and send the user planetunnel information corresponding to the PDU session and the uplink datatransmission rule to the user plane function network elementcorresponding to the PDN connection during network handover preparation.

According to the handover method provided in this embodiment of thisapplication, in a handover preparation phase, the first control planefunction network element determines the user plane tunnel informationcorresponding to the PDU session, and sends, to the user plane functionnetwork element corresponding to the PDN connection, the user planetunnel information corresponding to the PDU session. As such, an updateof the uplink data transmission rule due to a bearer newly added duringsession establishment/modification can be prevented.

With reference to the third aspect, in some possible implementations,before the sending, by the first control plane function network element,a first session management request to the user plane function networkelement corresponding to the PDN connection, the method further includesreceiving, by the first control plane function network element, a secondsession management request, where the second session management requestcarries first indication information, and the first indicationinformation is used to indicate to prepare for handing over the terminalfrom the second network to the first network.

With reference to the third aspect, in some possible implementations,the method further includes sending, by the first control plane functionnetwork element to the user plane function network element correspondingto the PDN connection, a downlink data transmission rule correspondingto the PDU session, where the downlink data transmission rule is used toindicate to send, through the user plane tunnel corresponding to the PDUsession, a downlink data packet received by the user plane functionnetwork element corresponding to the PDN connection.

In a possible implementation, the first control plane function networkelement may send a third session management request to the user planefunction network element corresponding to the PDN connection duringhandover execution, where the third session management request is usedto request to modify a downlink user plane tunnel corresponding to thePDU session, and the third session management request carries thedownlink data transmission rule.

In another possible implementation, the first control plane functionnetwork element may add the downlink data transmission rule and firstrule indication information to the first session management requestduring session establishment/modification, where the first ruleindication information is used to indicate not to enable the downlinkdata transmission rule. Correspondingly, the first control planefunction network element sends a third session management request to theuser plane function network element corresponding to the PDN connectionduring handover execution, where the third session management requestcarries second rule indication information, and the second ruleindication information is used to indicate to enable the downlink datatransmission rule.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element indicatesthe downlink data transmission rule to the user plane function networkelement corresponding to the PDN connection in a handover executionphase. As such, the user plane function network element corresponding tothe PDN connection matches received downlink data to a correspondingdownlink tunnel according to the downlink data transmission rule, andsends the received downlink data to a V-UPF or an access network (suchas a V-UPF or an access network in FIG. 1 or FIG. 2) in the firstnetwork through the downlink tunnel.

According to a fourth aspect, this application provides a communicationssystem. The system includes a first control plane function networkelement and a user plane function network element. The first controlplane function network element is configured to: determine at least onebearer that needs to be established when a PDU session of a terminal ina first network is to be switched to a PDN connection in a secondnetwork; obtain tunnel information of the PDN connection, where thetunnel information of the PDN connection includes user plane tunnelinformation corresponding to each of the at least one bearer; and send afirst session management request to the user plane function networkelement, where the first session management request is used to requestto establish or modify a user plane tunnel corresponding to each of theat least one bearer, the first session management request carries anuplink data transmission rule and the tunnel information of the PDNconnection, and the uplink data transmission rule is used to indicatethe user plane function network element corresponding to the PDU sessionhow to forward uplink data that is received through the user planetunnel corresponding to the PDN connection. The user plane functionnetwork element is configured to: receive the first session managementrequest from the first control plane function network element; and senda first session management response to the first control plane functionnetwork element based on the first session management request. The firstcontrol plane function network element is further configured to receivethe first session management response from the user plane functionnetwork element.

With reference to the fourth aspect, in some possible implementations,the first session management request further carries a downlinktransmission rule and first rule indication information, where thedownlink data transmission rule is used to indicate the user planefunction network element corresponding to the PDU session to forwardreceived downlink data through a corresponding user plane tunnel, andthe first rule indication information is used to indicate not to enablethe downlink data transmission rule.

Optionally, the first control plane function network element is furtherconfigured to send second rule indication information to the user planefunction network element, where the second rule indication informationis used to indicate to enable the downlink data transmission rule.

With reference to the fourth aspect, in some possible implementations,the first control plane function network element is further configuredto send the downlink data transmission rule to the user plane functionnetwork element.

According to a fifth aspect, an embodiment of this application providesa communications system. The system includes a first control planefunction network element and a user plane function network element. Thefirst control plane function network element is configured to: determineat least one bearer that needs to be established when a PDU session of aterminal in a first network is to be switched to a PDN connection in asecond network; and send a first session management request to the userplane function network element, where the first session managementrequest is used to request to establish or modify a user plane tunnelcorresponding to each of the at least one bearer, the first sessionmanagement request carries an uplink data transmission rule, and theuplink data transmission rule is used to indicate the user planefunction network element corresponding to the PDU session how to forwarduplink data that is received through the user plane tunnel correspondingto the PDN connection. The user plane function network element isconfigured to: receive the first session management request from thefirst control plane function network element, determine tunnelinformation of the PDN connection based on the first session managementrequest, where the tunnel information of the PDN connection includesuser plane tunnel information corresponding to each of the at least onebearer; and send a first session management response to the firstcontrol plane function network element, where the first sessionmanagement response carries the tunnel information of the PDNconnection. The first control plane function network element is furtherconfigured to receive the first session management response from theuser plane function network element.

With reference to the fifth aspect, in some possible implementations,the first session management request further carries a downlinktransmission rule and first rule indication information, where thedownlink data transmission rule is used to indicate the user planefunction network element corresponding to the PDU session to forwardreceived downlink data through a corresponding user plane tunnel, andthe first rule indication information is used to indicate not to enablethe downlink data transmission rule.

Optionally, the first control plane function network element is furtherconfigured to send second rule indication information to the user planefunction network element, where the second rule indication informationis used to indicate to enable the downlink data transmission rule.

With reference to the fifth aspect, in some possible implementations,the first control plane function network element further sends thedownlink data transmission rule to the user plane function networkelement.

According to a sixth aspect, an embodiment of this application providesa handover apparatus, including units configured to perform steps in thehandover method in any one of the first aspect or the implementations ofthe first aspect.

According to a seventh aspect, an embodiment of this applicationprovides a handover apparatus, including units configured to performsteps in the handover method in any one of the second aspect or theimplementations of the second aspect.

According to an eighth aspect, an embodiment of this applicationprovides a handover apparatus, including units configured to performsteps in the handover method in any one of the third aspect or theimplementations of the third aspect.

According to a ninth aspect, an embodiment of this application providesa handover device. The device may be the handover apparatus in theforegoing method design, or a chip configured in the handover apparatus.The device includes a processor, a memory, and a communicationsinterface. The processor, the memory, and the communications interfacemay implement communication using a bus, or may implement communicationin another manner such as wireless transmission. The memory isconfigured to store an instruction, and the processor is configured toexecute the instruction stored in the memory. The memory stores programcode, and the processor may invoke the program code stored in thememory, to perform the handover method in any one of the first aspect orthe possible implementations of the first aspect.

According to a tenth aspect, an embodiment of this application providesa handover device. The device may be the handover apparatus in theforegoing method design, or a chip configured in the handover apparatus.The device includes a processor, a memory, a communications interface,and a bus. The processor, the memory, and the communications interfacemay implement communication using the bus, or may implementcommunication in another manner such as wireless transmission. Thememory is configured to store an instruction, and the processor isconfigured to execute the instruction stored in the memory. The memorystores program code, and the processor may invoke the program codestored in the memory, to perform the handover method in any one of thesecond aspect or the possible implementations of the second aspect.

According to an eleventh aspect, an embodiment of this applicationprovides a handover device. The device may be the handover apparatus inthe foregoing method design, or a chip configured in the handoverapparatus. The device includes a processor, a memory, and acommunications interface. The processor, the memory, and thecommunications interface may implement communication using a bus, or mayimplement communication in another manner such as wireless transmission.The memory is configured to store an instruction, and the processor isconfigured to execute the instruction stored in the memory. The memorystores program code, and the processor may invoke the program codestored in the memory, to perform the handover method in any one of thethird aspect or the possible implementations of the third aspect.

According to a twelfth aspect, a chip system is provided. The chipsystem includes a processor, configured to implement functions includedin the foregoing aspects, for example, generating, receiving, sending,or processing data and/or information included in the foregoing methods.In a possible design, the chip system further includes a memory. Thememory is configured to store a program instruction and data that arenecessary for a handover apparatus. For example, the program instructionstored in the memory includes an instruction that is used to perform thehandover method in any one of the first to the third aspects or thepossible implementations of the first to the third aspects.

Optionally, the chip system may include a chip, or may include a chipand another discrete device.

According to a thirteenth aspect, a computer program product isprovided. The computer program product includes computer program code.When the computer program code runs on a computer, the computer isenabled to perform the handover method in any one of the first to thethird aspects or the possible implementations of the first to the thirdaspects.

According to a fourteenth aspect, a computer readable medium isprovided. The computer readable medium stores program code executed by ahandover apparatus, and the program code includes an instruction that isused to perform the handover method in any one of the first to the thirdaspects or the possible implementations of the first to the thirdaspects.

In this application, the implementations provided in the foregoingaspects can be further combined to provide more implementations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of a system architecture according to anembodiment of this application;

FIG. 2 is a schematic diagram of another system architecture accordingto an embodiment of this application;

FIG. 3 is a schematic flowchart of a handover method according to anembodiment of this application;

FIG. 4 is a schematic flowchart of another handover method according toan embodiment of this application;

FIG. 5 is a schematic flowchart of still another handover methodaccording to an embodiment of this application;

FIG. 6A, FIG. 6B, and FIG. 6C are schematic flowcharts of still anotherhandover method according to an embodiment of this application;

FIG. 7A, FIG. 7B, and FIG. 7C are schematic flowcharts of still anotherhandover method according to an embodiment of this application;

FIG. 8A and FIG. 8B are schematic flowcharts of still another handovermethod according to an embodiment of this application;

FIG. 9A, FIG. 9B, and FIG. 9C are schematic flowcharts of still anotherhandover method according to an embodiment of this application;

FIG. 10A, FIG. 10B, and FIG. 10C are schematic flowcharts of stillanother handover method according to an embodiment of this application;

FIG. 11A and FIG. 11B are schematic flowcharts of still another handovermethod according to an embodiment of this application;

FIG. 12A, FIG. 12B, and FIG. 12C are schematic flowcharts of stillanother handover method according to an embodiment of this application;

FIG. 13 is a schematic block diagram of a handover apparatus accordingto an embodiment of this application;

FIG. 14 is a schematic block diagram of another handover apparatusaccording to an embodiment of this application;

FIG. 15 is a schematic block diagram of still another handover apparatusaccording to an embodiment of this application;

FIG. 16 is a schematic block diagram of still another handover apparatusaccording to an embodiment of this application;

FIG. 17 is a schematic block diagram of still another handover apparatusaccording to an embodiment of this application; and

FIG. 18 is a schematic block diagram of still another handover apparatusaccording to an embodiment of this application.

DESCRIPTION OF EMBODIMENTS

The following describes technical solutions of this application withreference to accompanying drawings.

Application scenarios of embodiments of this application are firstdescribed with reference to FIG. 1 and FIG. 2. FIG. 1 and FIG. 2 arerespectively a schematic diagram of a system architecture 100 and aschematic diagram of a system architecture 200 according to embodimentsof this application. FIG. 1 shows an interworking system architecture100 of a 4G network and a 5G network in a non-roaming scenario, and FIG.2 shows an interworking system architecture 200 of a 4G network and a 5Gnetwork in a home routed roaming scenario.

For example, the system architecture 100 and the system architecture 200may include network elements in the 4G network and network elements inthe 5G network. Some modules in the system architecture 100 and thesystem architecture 200 have functions of a network element in the 4Gnetwork and a network element in the 5G network, for example, a userplane function (UPF)+PGW-U module, a session management function(SMF)+PDN gateway control plane function (PGW-C) module, a policycontrol function (PCF)+policy and charging rules function (PCRF) module,and a home subscriber server (HSS)+unified data management (UDM) module.

The UPF+PGW-U module is used for user data transmission management. Inthe interworking system architecture, the module can not only be usedfor 4G data transmission, but also provide a 5G data transmissionfunction.

The SMF+PGW-C module is used for session establishment, deletion, andmodification management. In the interworking system architecture, themodule can provide both a 4G session management function and a 5Gsession management function.

The PCF+PCRF module is used for policy and charging control. In theinterworking system architecture, the module can provide a terminal withboth 4G policy and charging control and 5G policy and charging control.

The HSS+UDM module is configured to store subscription data of a user.In the interworking system architecture, the module stores both 4Gsubscription information and 5G subscription information of a terminal.

It should be understood that, “+” indicates integrated configuration. AUPF is a user plane function of the 5G network, and a PGW-U is a gatewayuser plane function that corresponds to the UPF and that is of the 4Gnetwork; an SMF is a session management function of the 5G network, anda PGW-C is a gateway control plane function that corresponds to the SMFand that is in the 4G network; a PCF is a policy control function of the5G network, and a PCRF is a policy and charging rules function thatcorresponds to the PCF and that is of the 4G network. The “integratedconfiguration” herein means that one module may have functions of twonetwork functional entities.

In the embodiments of this application, for ease of description, anHSS+UDM entity is referred to as a user data management network element,an SMFHPGW-C is referred to as a control plane function network element,and a UPF+PGW-U is referred to as a user plane function network element.This is described herein for once, and details are not described belowagain. Certainly, the foregoing network device of integratedconfiguration may alternatively have other names. This is notspecifically limited in the embodiments of this application.

In addition, the system architecture 100 and the system architecture 200may further include an access and mobility management function (AMF)module of the 5G network, an MME module, and a serving gateway (SGW)module.

The MME module is used for mobility management of a user. For example,mobility management of the user mainly includes attachment management,accessibility management, mobility management, paging management, accessauthentication and authorization, and non-access stratum signalingencryption and integrity protection of the user.

The SGW module is a user plane gateway, and is a user plane terminationpoint of an evolved universal terrestrial radio access network(E-UTRAN). The SGW module manages data packet routing and transmission,adds a packet tag of a transport layer, and the like.

The AMF module is used for access and mobility management of a user,which mainly includes registration management, accessibility management,mobility management, paging management, access authentication andauthorization, non-access stratum signaling encryption and integrityprotection, and the like of the user.

The terminal accesses the 4G network using the E-UTRAN, and the terminalaccesses the 5G network using a next generation (NG) radio accessnetwork (NG-RAN).

The NG-RAN is used to provide the terminal with a radio air interface toaccess a core network, in order to obtain a corresponding service.

The E-UTRAN is used for managing radio resources, establishing,modifying, or deleting an air interface resource for the terminal,providing data and signaling transmission for the terminal, and thelike.

The following describes communications interfaces between the foregoingmodules in the system architecture 100 and the system architecture 200.

An S1-MME interface is a control plane interface between the MME and theE-UTRAN.

An S1-U interface is a user plane interface between the S-GW and theE-UTRAN.

An S5-U interface is a user plane interface between the SGW and thePGW-U, and is configured to transmit user plane data of a UE.

An S5-C interface is a control plane management interface between theSGW and the PGW-U, and is configured to establish an SGW and PGW-U userplane connection for a UE.

An S6a interface is an interface between the MME and the HSS, and isconfigured to obtain subscription data of a user and perform anauthentication and authorization function for UE.

An S11 interface is an interface between the SGW and the MIME, and isconfigured to establish a user plane bearer.

An N1 interface is an interface between a UE and the AMF, and is usedfor non-access stratum signaling management and transmission.

An N2 interface is an interface between the NG-RAN and the AMF, and isused for signaling transmission.

An N3 interface is an interface between the UPF and the NG-RAN, and isconfigured to transmit user data.

An N4 interface is an interface between the SMF and the UPF, and isconfigured to establish a user plane transmission channel.

An N7 interface is an interface between the SMF and the PCF, and isconfigured to work out and deliver policy control and charginginformation.

An N8 interface is an interface between the AMF and a UDM, and isconfigured to obtain mobility-related subscription information of a userand the like.

An N10 interface is an interface between the SMF and the UDM, and isconfigured to obtain session management-related subscription informationof a user and the like.

An N11 interface is an interface between the SMF and the AMF, and isconfigured to transmit session management information and the like.

An N15 interface is an interface between the AMF and the PCF, and isconfigured to obtain access and mobility-related policy information.

In addition, in the system architecture 200, an HPLMN indicates a localnetwork, and a VPLMN indicates a visited network or a roaming network.For example, the HPLMN indicates a home public land mobile network(HPLMN), and the VPLMN indicates a visited or roaming PLMN. For example,a v-SMF indicates an SMF in the roaming network and a v-UPF indicates aUPF in the roaming network.

It should be understood that, to support interworking of the 4G networkand the 5G network, a first interface is introduced into the systemarchitecture 100 and the system architecture 200. The first interface isa communications interface between the AMF in the 5G network and the MMEin the 4G network, and the first interface may be indicated using an N26interface. The system architecture optionally supports the N26interface. A handover process can be used only in an interworking systemarchitecture that supports the N26 interface, to ensure servicecontinuity.

It should be further understood that, because the embodiments of thisapplication include a process of handing over a terminal between the 4Gnetwork and the 5G network, the system architecture 100 and the systemarchitecture 200 support the N26 interface.

It should be noted that in the system architecture 100 and the systemarchitecture 200, names of the interfaces between the modules are onlyexamples, and the interfaces may have other names in implementation.This is not specifically limited in the embodiments of this application.

It should be further noted that in the system architecture 100 and thesystem architecture 200, the NG-RAN in the 5G network may also bereferred to as an access device. The access device is a device foraccessing a core network, and may be, for example, a base station, abroadband network gateway (BNG), an aggregation switch, or a non-3GPPaccess device. The base station may be in various forms, for example, amacro base station, a micro base station (also referred to as a smallcell), a relay station, or an access point. This is not limited in theembodiments of this application.

Certainly, in the system architecture 100 and the system architecture200, the 4G network and the 5G network may further include othermodules. For example, the 4G network may further include a generalpacket radio system (GPRS) serving GPRS support node (SGSN) module, andthe 5G network may further include an authentication server function(AUSF) module and a network slice selection function (NSSF) module. Thisis not limited in the embodiments of this application.

The terminal in the embodiments of this application may include ahandheld device, an in-vehicle device, a wearable device, and acomputing device that have a wireless communication function, or anotherprocessing device connected to a wireless modem, and may further includea subscriber unit, a cellular phone, a smartphone, a wireless data card,a personal digital assistant (PDA) computer, a tablet computer, awireless modem, a handheld device, a laptop computer, a cordless phoneor a wireless local loop (WLL) station, a machine type communication(MTC) terminal, a user equipment (UE), a mobile station (MS), a terminaldevice, and the like. For ease of description, all the foregoing devicesare collectively referred to as a terminal.

It should be understood that, as the terminal constantly moves, a changeof a location of the terminal may result in handover of the terminalbetween different networks. For example, when the terminal moves fromthe 4G network to the 5G network, a PDN connection in the 4G networkneeds to be switched to a PDU session in the 5G network, or when theterminal is handed over from the 5G network to the 4G network, a PDUsession in the 5G network needs to be switched to a PDN connection inthe 4G network.

In the 4G network, a control plane tunnel corresponding to the PDNconnection is allocated to the PDN connection. The control plane tunnelis used to transmit control signaling corresponding to the PDNconnection. The PDN connection includes at least one bearer, and a userplane tunnel corresponding to each of the at least one bearer isallocated to the bearer. The user plane tunnel corresponding to eachbearer is used to transmit at least one data flow included in thebearer.

In other words, one PDN connection corresponds to one control planetunnel, and one bearer in one PDN connection corresponds to one userplane tunnel.

In the 5G network, the PDU session includes at least one data flow. Auser plane tunnel corresponding to the PDU session is allocated to thePDU session. There is no control plane tunnel corresponding to the PDUsession in 5G.

In other words, no control plane tunnel is allocated to the PDU session,and one PDU session corresponds to one user plane tunnel.

Therefore, the following problems exist when the terminal is handed overbetween the two networks.

(1) When the terminal is handed over from the 4G network to the 5Gnetwork, there is a problem of how to map at least one user plane tunnel(for example, a plurality of user plane tunnels) corresponding to a PDNconnection to one user plane tunnel corresponding to a PDU session.

(2) When the terminal is handed over from the 5G network to the 4Gnetwork, there is a problem of how to map one user plane tunnelcorresponding to a PDU session to at least one user plane tunnel (forexample, a plurality of user plane tunnels) corresponding to a PDNconnection.

(3) When the terminal is handed over from the 5G network to the 4Gnetwork, there is a problem of how to establish a control plane tunnelcorresponding to a PDN connection in the 4G network.

The foregoing problems may result in a loss of a data packet in anetwork handover process, and consequently, normal transmission of thedata packet is affected.

A handover method and apparatus provided in the embodiments of thisapplication help prevent a loss of a data packet in a network handoverprocess, thereby ensuring normal transmission of the data packet.

FIG. 3 is a schematic flowchart of a handover method 300 according to anembodiment of this application. The method 300 may be applied to thesystem architecture shown in FIG. 1 or FIG. 2, or the method 300 may beapplied to another similar architecture. This is not limited in thisembodiment of this application.

It should be understood that, the method 300 may be performed by a firstcontrol plane function network element. The first control plane functionnetwork element may be, for example, the SMF-PGW-C shown in FIG. 1 orFIG. 2.

S310. The first control plane function network element determines atleast one bearer that needs to be established when a PDU session of aterminal in a first network is to be switched to a PDN connection in asecond network.

S320. The first control plane function network element obtains tunnelinformation of the PDN connection, where the tunnel information of thePDN connection includes user plane tunnel information corresponding toeach of the at least one bearer.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. It should befurther understood that, the PDU session in the first network isequivalent to the PDN connection in the second network.

Optionally, in S310, when determining that the PDU session in the firstnetwork needs to be established or modified, the first control planefunction network element may determine the at least one bearer thatneeds to be established when the PDU session of the terminal in thefirst network is to be switched to the PDN connection in the secondnetwork.

Optionally, in S310, the first control plane function network elementmay determine, in a plurality of cases, that the PDU session in thefirst network needs to be established or modified. This is not limitedin this embodiment of this application.

In a possible implementation, the first control plane function networkelement may receive a first request message from the AMF, where thefirst request message is used to request to establish the PDU session inthe first network, and the PDU session includes at least one data flow.Additionally, the first control plane function network elementdetermines, based on the first request message, that the PDU session inthe first network needs to be established.

In another possible implementation, the first control plane functionnetwork element may receive a second request message from anothernetwork element, where the second request message is used to trigger thefirst control plane function network element to modify the PDU sessionin the first network. For example, the second request message is used totrigger the first control plane function network element to add at leastone data flow to the PDU session.

In still another possible implementation, the first control planefunction network element may be triggered, by a process of the firstcontrol plane function network element, to modify the PDU session in thefirst network.

Optionally, the first control plane function network element maydetermine the at least one bearer based on the at least one data flowand an allocation policy.

Optionally, the at least one data flow may be, for example, a quality ofservice (QoS) flow. A QoS flow may be a guaranteed bit rate (GBR) QoSflow or a non-GBR QoS flow.

Optionally, the allocation policy may be pre-configured, or may beindicated by another network element using indication information. Thisis not limited in this embodiment of this application.

Optionally, specific content of the allocation policy is not limited inthis embodiment of this application. For example, the allocation policymay be: (1) all non-GBR QoS flows in one PDU session share a defaultbearer; (2) different GBR QoS flows in one PDU session use differentdedicated bearers; or (3) several different GBR QoS flows in one PDUsession share a dedicated bearer.

For example, during session establishment, when the at least one dataflow includes five GBR QoS flows and six non-GBR QoS flows, the firstcontrol plane function network element establishes one dedicated bearerfor each of the five GBR QoS flows, and establishes one public defaultbearer for the six non-GBR QoS flows.

For another example, during session modification, if a dedicated bearer1 and a dedicated bearer 2 have been established for the PDU sessionduring establishment of the PDU session, and the at least one data flowincludes one GBR QoS flow and one non-GBR QoS flow, the first controlplane function network element establishes a dedicated bearer 3 for theGBR QoS flow, and establishes a public default bearer for the non-GBRQoS flow.

For another example, during session modification, if a dedicated bearer1, a dedicated bearer 2, and a default bearer have been established forthe PDU session during establishment of the PDU session, and the atleast one data flow includes one GBR QoS flow and one non-GBR QoS flow,the first control plane function network element establishes a dedicatedbearer 3 for the GBR QoS flow, and the non-GBR QoS flow shares thedefault bearer with other non-GBR QoS flows in the default bearer.

Optionally, the user plane tunnel information corresponding to eachbearer in S320 may include a tunnel identifier of a user plane tunnelcorresponding to the bearer and other information about the user planetunnel, such as an Internet Protocol (IP) address corresponding to thetunnel. This is not limited in this embodiment of this application.

Optionally, in S320, the first control plane function network elementmay obtain, in a plurality of manners, the user plane tunnel informationcorresponding to each of the at least one bearer. This is not limited inthis embodiment of this application.

In a possible implementation, the first control plane function networkelement itself may allocate the user plane tunnel information to each ofthe at least one bearer.

In another possible implementation, the first control plane functionnetwork element may request a user plane function network elementcorresponding to the PDU session to allocate user plane tunnelinformation to each of the at least one bearer, and obtain the userplane tunnel information corresponding to each bearer from the userplane function network element corresponding to the PDU session.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element determinesthe at least one bearer that needs to be established when the PDUsession of the terminal in the first network is to be switched to thePDN connection in the second network, and obtains the user plane tunnelinformation corresponding to each of the at least one bearer. As such,the first control plane function network element establishes a userplane tunnel corresponding to a bearer in the PDN connection when thePDU session is to be switched to the PDN connection. This helps preventa loss of a data packet in the bearer included in the PDN connection ina network handover process, thereby ensuring normal transmission of thedata packet.

It should be understood that, the first control plane function networkelement and the user plane function network element corresponding to thePDU session in this embodiment may be respectively the SMF+PGW-C and theUPF+PGW-U in the system architecture shown in FIG. 1 or FIG. 2.

Optionally, the method 300 further includes sending, by the firstcontrol plane function network element, an uplink data transmission ruleto the user plane function network element corresponding to the PDUsession, where the uplink data transmission rule is used to indicate theuser plane function network element corresponding to the PDU session howto forward uplink data that is received through the user plane tunnelcorresponding to the PDN connection. For example, the uplink datatransmission rule is used to indicate the user plane function networkelement corresponding to the PDU session to send an uplink data packetto an external data network over a specified forwarding path.

In a possible implementation, the first control plane function networkelement may send a first session management request to the user planefunction network element corresponding to the PDU session, where thefirst session management request is used to request to establish ormodify the user plane tunnel corresponding to each of the at least onebearer, and the first session management request carries the uplink datatransmission rule.

Correspondingly, the user plane function network element correspondingto the PDU session may receive the first session management request fromthe first control plane function network element, and send a firstsession management response to the first control plane function networkelement based on the first session management request.

Optionally, the first session management request may be an N4 sessionmanagement request, and the first session management response may be anN4 session management response.

It should be understood that in this embodiment of this application, theN4 session management request may be an N4 session establishment requestor an N4 session modification request transmitted, through an N4interface, between the first control plane function network element andthe user plane function network element corresponding to the PDUsession. Correspondingly, the N4 session management response may be anN4 session establishment response or an N4 session modification responsetransmitted, through the N4 interface, between the first control planefunction network element and the user plane function network elementcorresponding to the PDU session.

Optionally, when the first control plane network element itselfallocates the user plane tunnel information to each of the at least onebearer, the first control plane function network element may add theuser plane tunnel information corresponding to each of the at least onebearer to the first session management request. Additionally, when thefirst control plane function network element requests the user planefunction network element corresponding to the PDU session to allocateuser plane tunnel information to each of the at least one bearer, theuser plane function network element corresponding to the PDU session mayadd the user plane tunnel information corresponding to each of the atleast one bearer to the first session management response.

It should be noted that, after learning of the user plane tunnelinformation corresponding to each of the at least one bearer and anuplink data transmission rule corresponding to the bearer, the userplane function network element corresponding to the PDU session mayestablish an uplink user plane tunnel of the bearer on the user planefunction network element corresponding to the PDU session.

Optionally, in S320, the tunnel information of the PDN connection mayfurther include control plane tunnel information corresponding to thePDN connection.

Correspondingly, the first control plane function network elementfurther needs to obtain the control plane tunnel informationcorresponding to the PDN connection.

It should be understood that, a control plane tunnel corresponding tothe PDN connection is used to transmit control signaling of the PDNconnection. In other words, a granularity of the control plane tunnelcorresponding to the PDN connection is per PDN connection basis.

Optionally, the first control plane function network element may obtain,in a plurality of manners, the control plane tunnel informationcorresponding to the PDN connection. This is not limited in thisembodiment of this application.

In a possible implementation, the first control plane function networkelement may allocate the control plane tunnel information to the PDNconnection during establishment of the PDU session.

Optionally, the control plane tunnel information corresponding to thePDN connection may include a tunnel identifier of the control planetunnel corresponding to the PDN connection and other information aboutthe control plane tunnel, such as an IP address corresponding to thecontrol plane tunnel. This is not limited in this embodiment of thisapplication.

According to the handover method provided in this embodiment of thisapplication, the tunnel information corresponding to the PDN connectionfurther includes the control plane tunnel information corresponding tothe PDN connection, such that the first control plane function networkelement establishes the control plane tunnel corresponding to the PDNconnection corresponding to the PDU session. This helps prevent a lossof control signaling of the PDN connection in the network handoverprocess, thereby ensuring normal transmission of the control signaling.

Optionally, in S320, the first control plane function network elementmay obtain the tunnel information of the PDN connection during sessionestablishment/modification. Alternatively, the first control planefunction network element may obtain the tunnel information of the PDNconnection during handover preparation.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element obtainsthe tunnel information of the PDN connection during establishment ormodification of the PDU session or during handover preparation. As such,the first control plane function network element establishes, beforehandover execution, the user plane tunnel corresponding to the bearer inthe PDN connection and the control plane tunnel corresponding to the PDNconnection, thereby reducing a delay in the handover process.

Optionally, the method 300 further includes obtaining, by the firstcontrol plane function network element, a bearer identifiercorresponding to each of the at least one bearer, such that the firstcontrol plane function network element associates the bearer identifiercorresponding to each bearer with the user plane tunnel corresponding toeach bearer.

It should be understood that, the bearer identifier corresponding toeach of the at least one bearer is allocated by the mobility managementnetwork element, such that the bearer identifier corresponding to eachbearer may be unique in the PDU session. In other words, the beareridentifier corresponding to each bearer may uniquely identify a userplane tunnel corresponding to the bearer.

In addition, the control plane tunnel information corresponding to thePDN connection may uniquely identify the PDN connection.

The following describes, with reference to the application scenariosshown in FIG. 1 and FIG. 2, a process in which the first control planefunction network element obtains the bearer identifier corresponding toeach bearer.

(1) In the Application Scenario Shown in FIG. 1

In a possible implementation, the first control plane function networkelement may send a bearer identifier request to the mobility managementnetwork element, where the bearer identifier request is used to allocatea bearer identifier to each of the at least one bearer. Correspondingly,the mobility management network element receives the bearer identifierrequest from the first control plane function network element, allocatesa bearer identifier to each bearer based on the bearer identifierrequest, and sends the bearer identifier corresponding to each of the atleast one bearer to the first control plane function network element.

It should be noted that, the first control plane function networkelement may store, into a bearer context corresponding to the PDNconnection, the user plane tunnel information corresponding to each ofthe at least one bearer, the bearer identifier corresponding to eachbearer, and the control plane tunnel information corresponding to thePDN connection that are obtained, and send the bearer context to themobility management network element during handover preparation.

Correspondingly, the mobility management network element may establish,based on the bearer context corresponding to the PDN connection, anuplink tunnel from a serving gateway (such as the SGW in FIG. 1) in thesecond network to the user plane function network element correspondingto the PDU session.

(2) In the Application Scenario Shown in FIG. 2

It should be understood that, in FIG. 2, a second control plane functionnetwork element is a visited control plane network element correspondingto the PDU session, and the first control plane function network elementis a home control plane network element corresponding to the PDUsession. Therefore, the first control plane function network elementneeds to obtain the bearer identifier corresponding to each bearer usingthe second control plane function network element, and the secondcontrol plane function network element is responsible for networkhandover of the terminal.

In a possible implementation, the first control plane function networkelement sends a bearer identifier request to the mobility managementnetwork element using the second control plane function network element,where the bearer identifier request is used to request to allocate abearer identifier to each of the at least one bearer. Additionally, thefirst control plane function network element receives the beareridentifier corresponding to each of the at least one bearer from themobility management network element using the second control planefunction network element.

It should be noted that, after obtaining the user plane tunnelinformation corresponding to each of the at least one bearer, the beareridentifier corresponding to each bearer, and the control plane tunnelinformation corresponding to the PDN connection, the first control planefunction network element may send the user plane tunnel informationcorresponding to each of the at least one bearer, the bearer identifiercorresponding to each bearer, and the control plane tunnel informationcorresponding to the PDN connection to the second control plane functionnetwork element during establishment/modification of the PDU session.

Correspondingly, the second control plane function network element maystore, into the bearer context of the PDN connection, the user planetunnel information corresponding to each of the at least one bearer, thebearer identifier corresponding to each bearer, and the control planetunnel information corresponding to the PDN connection that are obtainedfrom the first control plane function network element.

In another possible implementation, the first control plane functionnetwork element may send the tunnel information of the PDN connection tothe second control plane function network element, and obtain the beareridentifier corresponding to each of the at least one bearer from thesecond control plane function network element. Correspondingly, thesecond control plane function network element may store the beareridentifier corresponding to each of the at least one bearer.

In other words, the first control plane function network element sendsthe tunnel information of the PDN connection to the second control planefunction network element, and the second control plane function networkelement determines, based on the tunnel information of the PDNconnection, whether the mobility management network element needs to berequested to allocate a bearer identifier to the at least one bearer,and sends the bearer identifier request to the mobility managementnetwork element when the mobility management network element needs to berequested to allocate the bearer identifier to the at least one bearer.

For example, the first control plane function network element firstchecks whether a bearer identifier associated with the user plane tunnelinformation corresponding to each bearer exists in the obtained tunnelinformation of the PDN connection. If no bearer identifiers areallocated to some of the at least one bearer, the first control planefunction network element requests the mobility management networkelement to allocate bearer identifiers to the bearers.

It should be noted that, the second control plane function networkelement may store, into the bearer context of the PDN connection, theuser plane tunnel information corresponding to each of the at least onebearer, the bearer identifier corresponding to each bearer, and thecontrol plane tunnel information corresponding to the PDN connection.

In the two implementations in the application scenario shown in FIG. 2,because the bearer context corresponding to the PDN connection stored bythe second control plane function network element includes the userplane tunnel information corresponding to each of the at least onebearer, the bearer identifier corresponding to each bearer, and thecontrol plane tunnel information corresponding to the PDN connection,the mobility management network element may obtain the bearer contextcorresponding to the PDN connection from the second control planefunction network element during handover preparation, and establish,based on the bearer context corresponding to the PDN connection, theuplink tunnel from the serving gateway (such as the SGW in FIG. 1 orFIG. 2) in the second network to the user plane function network elementcorresponding to the PDU.

According to the handover method provided in this embodiment of thisapplication, because the second control plane function network elementis a visited control plane network element corresponding to the PDUsession, compared with obtaining the bearer context corresponding to thePDN connection from the first control plane function network element,obtaining, by the mobility management network element, the bearercontext corresponding to the PDN connection from the second controlplane function network element can reduce a delay generated in thehandover process.

Optionally, the method 300 further includes sending, by the firstcontrol plane function network element to the user plane functionnetwork element corresponding to the PDU session, a downlink datatransmission rule corresponding to the PDN connection, where thedownlink data transmission rule is used to indicate the user planefunction network element corresponding to the PDU session to forwardreceived downlink data through a corresponding user plane tunnel.

In a possible implementation, the first control plane function networkelement may send a third session management request to the user planefunction network element corresponding to the PDU session duringhandover execution, where the third session management request is usedto request to establish or modify a downlink user plane tunnelcorresponding to each of the at least one bearer, and the third sessionmanagement request carries the downlink data transmission rule.

In another possible implementation, the first control plane functionnetwork element may add the downlink data transmission rule and firstrule indication information to the first session management requestduring PDU session request/modification, where the first rule indicationinformation is used to indicate not to enable the downlink datatransmission rule. Correspondingly, the first control plane functionnetwork element sends a third session management request to the userplane function network element corresponding to the PDU session duringhandover execution, where the third session management request carriessecond rule indication information, and the second rule indicationinformation is used to indicate to enable the downlink data transmissionrule.

Optionally, the third session management request may be an N4 sessionmanagement request, and a third session management response may be an N4session management response.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element indicatesthe downlink data transmission rule to the user plane function networkelement corresponding to the PDU session in a handover execution phase,As such, the user plane function network element corresponding to thePDU session matches received downlink data to a corresponding downlinktunnel according to the downlink data transmission rule, and sends thereceived downlink data to the serving gateway (such as the SGW in FIG. 1or FIG. 2) in the second network through the downlink tunnel.

The following describes, in detail with reference to FIG. 4, a functionof the second control plane function network element in the embodimentof the foregoing method 300.

FIG. 4 is a schematic flowchart of a handover method 400 according to anembodiment of this application. The method 400 may be applied to thesystem architecture shown in FIG. 2, or the method 400 may be applied toanother similar architecture. This is not limited in this embodiment ofthis application.

It should be understood that, the method 400 may be performed by asecond control plane function network element, and the second controlplane function network element may be, for example, the v-SMF shown inFIG. 2.

S410. The second control plane function network element receives tunnelinformation of a PDN connection from a first control plane functionnetwork element, where the tunnel information of the PDN connectionincludes user plane tunnel information corresponding to each of at leastone bearer, the second control plane function network element is avisited control plane network element corresponding to the PDNconnection, and the first control plane function network element is ahome control plane network element corresponding to the PDN connection.Correspondingly, the first control plane function network element sendsthe tunnel information of the PDN connection to the second control planefunction network element.

S420. The second control plane function network element sends the tunnelinformation of the PDN connection to a mobility management networkelement.

Optionally, the tunnel information of the PDN connection may includecontrol plane tunnel information corresponding to the PDN connection.

In a possible implementation, the method 400 further includes:forwarding, by the second control plane function network element to themobility management network element, a bearer identifier request that issent by the first control plane function network element to the mobilitymanagement network element, where the bearer identifier request is usedto request to allocate a bearer identifier to each of the at least onebearer; and forwarding, to the first control plane function networkelement, the bearer identifier that corresponds to each of the at leastone bearer and that is sent by the mobility management network elementto the first control plane function network element.

In other words, the second control plane function network element has afunction of transparently transmitting a message transmitted between thefirst control plane function network element and a user plane functionnetwork element corresponding to the PDU session.

Optionally, the second control plane function network element mayfurther receive the bearer identifier corresponding to each of the atleast one bearer from the first control plane function network element.

In this way, a context corresponding to the PDN connection stored by thesecond control plane function network element may include the tunnelinformation of the PDN connection and the bearer identifiercorresponding to each of the at least one bearer.

In another possible implementation, the method 400 further includes:sending, by the second control plane function network element, a beareridentifier request to the mobility management network element based onthe tunnel information of the PDN connection, where the beareridentifier request is used to request to allocate a bearer identifier toeach of the at least one bearer included in the PDN connection; andreceiving, by the second control plane function network element, thebearer identifier corresponding to each of the at least one bearer fromthe mobility management network element.

For example, the second control plane network element may check whethera bearer identifier associated with the user plane tunnel informationcorresponding to each bearer exists in the tunnel information of the PDNconnection. If no bearer identifier is allocated to the at least onebearer, the second control plane function network element requests themobility management network element to allocate the bearer identifier toeach of the at least one bearer to which no bearer identifier isallocated.

In other words, the second control plane function network element mayhave a processing function.

Optionally, the second control plane function network element may storethe bearer identifier corresponding to each of the at least one bearer,and send the bearer identifier corresponding to each of the at least onebearer to the first control plane function network element.

In this way, the context corresponding to the PDN connection stored bythe second control plane function network element may include the tunnelinformation of the PDN connection and the bearer identifiercorresponding to each of the at least one bearer.

According to the handover method provided in this embodiment of thisapplication, the bearer context corresponding to the PDN connectionstored by the second control plane function network element includes thetunnel information of the PDN connection and the bearer identifiercorresponding to each of the at least one bearer. As such, the mobilitymanagement network element may directly obtain the bearer contextcorresponding to the PDN connection from the second control planefunction network element during handover preparation, and establish,based on the bearer context corresponding to the PDN connection, anuplink tunnel from a serving gateway (such as the SGW in FIG. 1 or FIG.2) in the second network to the user plane function network elementcorresponding to the PDU.

In addition, because the second control plane function network elementis a visited control plane network element corresponding to the PDUsession, compared with obtaining the bearer context corresponding to thePDN connection from the first control plane function network element,obtaining, by the mobility management network element, the bearercontext corresponding to the PDN connection from the second controlplane function network element can reduce a delay generated in ahandover process.

FIG. 5 is a schematic flowchart of a handover method 500 according to anembodiment of this application. The method 500 may be applied to thesystem architecture shown in FIG. 1 or FIG. 2, or the method 500 may beapplied to another similar architecture. This is not limited in thisembodiment of this application.

It should be understood that, the method 500 may be performed by a firstcontrol plane function network element. The first control plane functionnetwork element may be, for example, the SMF-PGW-C shown in FIG. 1 orFIG. 2.

S510. The first control plane function network element determines that aPDN connection of a terminal in a second network needs to be switched toa PDU session in a first network.

S520. The first control plane function network element obtains userplane tunnel information corresponding to the PDU session.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. It should befurther understood that, the PDU session in the first network isequivalent to the PDN connection in the second network.

Optionally, in S510, the first control plane function network elementmay determine, in a plurality of manners, that the PDN connection of theterminal in the second network needs to be switched to the PDU sessionin the first network. This is not limited in this embodiment of thisapplication.

(1) During Session Establishment/Modification

In a possible implementation, the first control plane function networkelement may receive a third request message, where the third requestmessage is used to request to establish the PDN connection in the secondnetwork. Additionally, the first control plane function network elementdetermines, based on the third request message, that the PDN connectionof the terminal in the second network needs to be switched to the PDUsession in the first network.

(2) During Handover Preparation

In a possible implementation, the first control plane function networkelement may receive a second session management request, where thesecond session management request carries first indication information,and the first indication information is used to indicate to prepare forhanding over the terminal from the second network to the first network.Additionally, the first control plane function network element maydetermine, based on the first indication information, that the PDNconnection in the second network needs to be switched to the PDU sessionin the first network.

Optionally, the first control plane function network element may receivethe second session management request from the AMF or receive the secondsession management request from the v-SMF.

Optionally, in S520, the first control plane function network elementmay obtain, in a plurality of manners, the user plane tunnel informationcorresponding to the PDU session. This is not limited in this embodimentof this application.

In a possible implementation, the first control plane function networkelement itself may allocate, to the PDU session, the user plane tunnelinformation corresponding to the PDU session.

In another possible implementation, the first control plane functionnetwork element requests a user plane function network elementcorresponding to the PDN connection to allocate user plane tunnelinformation corresponding to the PDU session, and obtains the user planetunnel information that corresponds to the PDU session from the userplane function network element corresponding to the PDN connection.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element determinesthat the PDN connection of the terminal in the second network needs tobe switched to the PDU session in the first network, and obtains theuser plane tunnel information corresponding to the PDU session, suchthat the first control plane function network element establishes a userplane tunnel corresponding to the PDU session when the PDN connection isto be switched to the PDU session. This helps prevent a loss of a datapacket in the PDU session in a network handover process, therebyensuring normal transmission of the data packet.

It should be understood that, the user plane function network elementcorresponding to the PDN connection may be the UPF+PGW-U in the systemarchitecture shown in FIG. 1 or FIG. 2.

Optionally, the method 500 further includes sending, by the firstcontrol plane function network element, an uplink data transmission ruleto the user plane function network element corresponding to the PDNconnection, where the uplink data transmission rule is used to indicatethe user plane function network element corresponding to the PDNconnection how to forward uplink data that is received through the userplane tunnel corresponding to the PDU session. For example, the uplinkdata transmission rule is used to indicate the user plane functionnetwork element corresponding to the PDN connection to send an uplinkdata packet to an external data network over a specified forwardingpath.

In a possible implementation, the first control plane function networkelement sends a first session management request to the user planefunction network element corresponding to the PDN connection, where thefirst session management request is used to request to establish ormodify the user plane tunnel corresponding to the PDU session, and thefirst session management request carries the uplink data transmissionrule.

Correspondingly, the user plane function network element correspondingto the PDN connection may receive the first session management requestfrom the first control plane function network element, and send a firstsession management response to the first control plane function networkelement based on the first session management request.

Optionally, the first session management request may be an N4 sessionmanagement request, and the first session management response may be anN4 session management response.

It should be understood that in this embodiment of this application, theN4 session management request may be an N4 session establishment requestor an N4 session modification request transmitted, through an N4interface, between the first control plane function network element andthe user plane function network element corresponding to the PDNconnection. Correspondingly, the N4 session management response may bean N4 session establishment response or an N4 session modificationresponse transmitted, through the N4 interface, between the firstcontrol plane function network element and the user plane functionnetwork element corresponding to the PDN connection.

Optionally, when the first control plane network element itselfallocates the user plane tunnel information to the PDU session, thefirst control plane function network element may add the user planetunnel information corresponding to the PDU session to the first sessionmanagement request. Additionally, when the first control plane functionnetwork element requests the user plane function network elementcorresponding to the PDN connection to allocate user plane tunnelinformation to the PDU session, the user plane function network elementcorresponding to the PDN connection may add the user plane tunnelinformation corresponding to the PDU session to the first sessionmanagement response.

Optionally, during session modification, the PDN connection in thesecond network may be updated, for example, some bearers are newlyestablished. Therefore, the user plane function network elementcorresponding to the PDN connection needs to know an updated uplink datatransmission rule, in order to associate data flows corresponding tothese bearers with the user plane tunnel corresponding to the PDUsession.

In a possible implementation, the first control plane function networkelement may receive a fourth request message from another networkelement, where the fourth request message triggers modification of thePDN connection in the second network. For example, the fourth requestmessage triggers addition of at least one bearer to the PDN connection.

In still another possible implementation, the first control planefunction network element may be triggered, by a process of the firstcontrol plane function network element, to modify the PDN connection inthe second network.

Optionally, the method 500 further includes: when determining that theuplink data transmission rule changes, sending, by the first controlplane function network element, an updated uplink data transmission ruleto the user plane function network element corresponding to the PDNconnection.

Optionally, when the first control plane function network element itselfallocates the user plane tunnel information to the PDU session, thefirst control plane function network element may obtain the user planetunnel information corresponding to the PDU session, and send, to theuser plane function network element corresponding to the PDN connection,the user plane tunnel information corresponding to the PDU session in asame phase; or obtain the user plane tunnel information corresponding tothe PDU session, and send, to the user plane function network elementcorresponding to the PDN connection, the user plane tunnel informationcorresponding to the PDU session in different phases. This is notlimited in this embodiment of this application.

In a possible implementation, the first control plane function networkelement may obtain the user plane tunnel information corresponding tothe PDU session and the uplink data transmission rule during sessionestablishment/modification, and send the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleto the user plane function network element corresponding to the PDNconnection during session establishment/modification.

In another possible implementation, the first control plane functionnetwork element may obtain the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleduring network handover preparation, and send the user plane tunnelinformation corresponding to the PDU session and the uplink datatransmission rule to the user plane function network elementcorresponding to the PDN connection during network handover preparation.

In still another possible implementation, the first control planefunction network element may obtain the user plane tunnel informationcorresponding to the PDU session and the uplink data transmission ruleduring session establishment/modification, and send the user planetunnel information corresponding to the PDU session and the uplink datatransmission rule to the user plane function network elementcorresponding to the PDN connection during network handover preparation.

According to the handover method provided in this embodiment of thisapplication, in a handover preparation phase, the first control planefunction network element determines the user plane tunnel informationcorresponding to the PDU session, and sends, to the user plane functionnetwork element corresponding to the PDN connection, the user planetunnel information corresponding to the PDU session. As such, an updateof the uplink data transmission rule due to a bearer newly added duringsession establishment/modification can be prevented.

Optionally, the method 500 further includes sending, by the firstcontrol plane function network element to the user plane functionnetwork element corresponding to the PDN connection, a downlink datatransmission rule corresponding to the PDU session, where the downlinkdata transmission rule is used to indicate to send, through the userplane tunnel corresponding to the PDU session, a downlink data packetreceived by the user plane function network element corresponding to thePDN connection.

In a possible implementation, the first control plane function networkelement may send a third session management request to the user planefunction network element corresponding to the PDN connection duringhandover execution, where the third session management request is usedto request to modify a downlink user plane tunnel corresponding to thePDU session, and the third session management request carries thedownlink data transmission rule.

In another possible implementation, the first control plane functionnetwork element may add the downlink data transmission rule and firstrule indication information to the first session management requestduring session establishment/modification, where the first ruleindication information is used to indicate not to enable the downlinkdata transmission rule. Correspondingly, the first control planefunction network element sends a third session management request to theuser plane function network element corresponding to the PDN connectionduring handover execution, where the third session management requestcarries second rule indication information, and the second ruleindication information is used to indicate to enable the downlink datatransmission rule.

Optionally, the third session management request may be an N4 sessionmanagement request, and the third session management response may be anN4 session management response.

According to the handover method provided in this embodiment of thisapplication, the first control plane function network element indicatesthe downlink data transmission rule to the user plane function networkelement corresponding to the PDN connection in a handover executionphase. As such, the user plane function network element corresponding tothe PDN connection matches received downlink data to a correspondingdownlink tunnel according to the downlink data transmission rule, andsends the received downlink data to a V-UPF or an access network (suchas the V-UPF or the access network in FIG. 1 or FIG. 2) in the firstnetwork through the downlink tunnel.

FIG. 6A, FIG. 6B, and FIG. 6C are a schematic flowchart of a handovermethod 600 according to an embodiment of this application. The method600 may be applied to the system architecture in the non-roamingscenario shown in FIG. 1, or the method 600 may be applied to anothersimilar architecture.

It should be understood that, the method 600 includes three phases intotal: a session establishment/modification phase, a handoverpreparation phase, and a handover execution phase. The followingdescribes the three phases in detail with reference to FIG. 6A, FIG. 6B,and FIG. 6C.

1. Session Establishment/Modification Phase

It should be noted that, S601 and S602 are a process in which theSMF+PGW-C determines at least one bearer that needs to be establishedwhen a PDU session of a terminal in a first network is to be switched toa PDN connection in a second network.

S601. The SMF+PGW-C receives a session establishment/modificationrequest from the AMF, where the session establishment/modificationrequest is used to request to establish the PDU session of the terminalin the first network or trigger modification of the PDU session in thefirst network. Correspondingly, the AMF sends the sessionestablishment/modification request to the SMF+PGW-C.

S602. The SMF+PGW-C determines, based on the sessionestablishment/modification request in S601, the at least one bearer thatneeds to be established when the PDU session of the terminal in thefirst network is to be switched to the PDN connection in the secondnetwork.

It should be noted that, S603 a to S605 a or S603 b to S605 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a user planetunnel corresponding to each of the at least one bearer.

It should be understood that, that the SMF+PGW-C and the UPF+PGW-Uestablish the user plane tunnel corresponding to each of the at leastone bearer may include obtaining, by the SMF+PGW-C and the UPF+PGW-U,user plane tunnel information corresponding to each bearer and an uplinkdata packet transmission rule.

Optionally, user plane tunnel information corresponding to each of theat least one bearer may be allocated by the SMF+PGW-C or the UPF+PGW-U.This is not limited in this embodiment of this application.

If the SMF+PGW-C allocates the user plane tunnel information, S603 a toS605 a are performed. If the UPF+PGW-U allocates the user plane tunnelinformation, S603 b to S605 b are performed.

(1) The SMF+PGW-C allocates the user plane tunnel information to eachbearer.

S603 a. The SMF+PGW-C allocates the user plane tunnel information toeach bearer.

S604 a. The SMF+PGW-C sends an N4 session establishment/modificationrequest to the UPF+PGW-U, where the N4 sessionestablishment/modification request is used to request to establish theuser plane tunnel corresponding to each of the at least one bearer, theN4 session establishment/modification request carries the user planetunnel information corresponding to each bearer and the uplink datapacket transmission rule, and the uplink data transmission rule is usedto indicate a user plane function network element (such as the UPF+PGW-Uin FIG. 6A, FIG. 6B, and FIG. 6C) corresponding to the PDU session howto forward uplink data that is received through the user plane tunnelcorresponding to the PDN connection. Correspondingly, the UPF+PGW-Ureceives the N4 session establishment/modification request from theSMF+PGW-C.

S605 a. The UPF+PGW-U sends an N4 session establishment/modificationresponse to the SMF+PGW-C based on the N4 sessionestablishment/modification request received in S604 a. Correspondingly,the SMF+PGW-C receives the N4 session establishment/modificationresponse from the UPF+PGW-U.

(2) The UPF+PGW-U allocates the user plane tunnel information to eachbearer.

S603 b. The SMF+PGW-C sends an N4 session establishment/modificationrequest to the UPF+PGW-U, where the N4 sessionestablishment/modification request is used to request to establish theuser plane tunnel corresponding to each of the at least one bearer, theN4 session establishment/modification request carries the uplink datapacket transmission rule, and the uplink data transmission rule is usedto indicate a user plane function network element (such as the UPF+PGW-Uin FIG. 6A, FIG. 6B, and FIG. 6C) corresponding to the PDU session howto forward uplink data that is received through the user plane tunnelcorresponding to the PDN connection. Correspondingly, the UPF+PGW-Ureceives the N4 session establishment/modification request from theSMF-PGW-C.

S604 b. The UPF+PGW-U allocates the user plane tunnel information toeach bearer based on the N4 session establishment/modification requestreceived in S603 b.

S605 b. The UPF+PGW-U sends an N4 session establishment/modificationresponse to the SMF+PGW-C, where the N4 sessionestablishment/modification response carries the user plane tunnelinformation corresponding to each bearer. Correspondingly, the SMF+PGW-Creceives the N4 session establishment/modification response from theUPF+PGW-U.

After S603 a or S605 b, the SMF-PGW-C stores the user plane tunnelinformation corresponding to each of the at least one bearer and theuplink data transmission rule into a bearer context corresponding to thePDN connection.

It should be further understood that, the user plane tunnel informationcorresponding to each bearer may include an identifier of the user planetunnel corresponding to each bearer.

Optionally, the user plane tunnel information corresponding to eachbearer may further include an IP address of the user plane tunnelcorresponding to each bearer, or other information about the user planetunnel corresponding to each bearer. This is not limited in thisembodiment of this application.

It should be noted that, S606 is a process in which the SMF-PGW-Cestablishes a control plane tunnel corresponding to the PDN connectionfor the PDN connection.

It should be understood that, that the SMF+PGW-C establishes the controlplane tunnel corresponding to the PDN connection for the PDN connectionmay be understood as that the SMF+PGW-C allocates, to the PDNconnection, control plane tunnel information corresponding to the PDNconnection.

S606. The SMF+PGW-C obtains the control plane tunnel informationcorresponding to the PDN connection.

After S606, the SMF-PGW-C stores the control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

It should be understood that, the control plane tunnel corresponding tothe PDN connection is used to transmit control signaling of the PDNconnection to the UPF+PGW-U. In other words, a granularity of thecontrol plane tunnel corresponding to the PDN connection is per PDNconnection basis. Therefore, S606 is performed only in a sessionestablishment phase, that is, S606 is not performed in a sessionmodification phase.

It should be further understood that, the control plane tunnelinformation corresponding to the PDN connection may include anidentifier of the control plane tunnel corresponding to the PDNconnection.

Optionally, the control plane tunnel information corresponding to thePDN connection may further include an IP address of the control planetunnel corresponding to the PDN connection, or other information aboutthe control plane tunnel corresponding to the PDN connection. This isnot limited in this embodiment of this application.

It should be noted that, S607 to S609 are a process in which theSMF+PGW-C obtains a bearer identifier corresponding to each of the atleast one bearer.

S607. The SMF+PGW-C sends a bearer update request to the AMF, where thebearer update request is used to request the bearer identifiercorresponding to each of the at least one bearer. Correspondingly, theAMF receives the bearer update request from the SMF+PGW-C.

S608. The AMF allocates the bearer identifier to each of the at leastone bearer based on the bearer update request.

S609. The AMF sends a bearer update response to the SMF+PGW-C, where thebearer update response carries the bearer identifier corresponding toeach of the at least one bearer. Correspondingly, the SMF+PGW-C receivesthe bearer update response from the AMF.

After S609, the SMF-PGW-C stores the bearer identifier of each of the atleast one bearer into the bearer context corresponding to the PDNconnection.

It should be understood that, the bearer identifier of each bearer mayidentify a user plane tunnel corresponding to the bearer.

Optionally, there is no sequence among the process of S603 a to S605 a(or S603 b to S605 b), the process of S606, and the process of S607 toS609. This is not limited in this embodiment of this application.

Based on the foregoing description, after S601 a (or S601 b) to S609,the bearer context corresponding to the PDN connection stored by theSMF+PGW-C may include the control plane tunnel information correspondingto the PDN connection, the user plane tunnel information correspondingto each of the at least one bearer, and the bearer identifiercorresponding to each of the at least one bearer.

Optionally, the bearer context corresponding to the PDN connection mayfurther include other information that is needed in the subsequenthandover preparation phase and handover execution phase, for example, aQoS parameter of each bearer, where the QoS parameter of each bearerincludes at least one of a QoS class identifier (QCI), an allocationretention priority (ARP), a packet filter, or a packet filter priority.This is not limited in this embodiment of this application.

2. Handover Preparation Phase

S610. The AMF receives a handover request from a source access network(such as a gNB in FIG. 6A, FIG. 6B, and FIG. 6C) of the first network,where the handover request carries information about a target accessnetwork (such as an Evolved Node B (eNB) in FIG. 6A, FIG. 6B, and FIG.6C). Correspondingly, the source access network sends the handoverrequest to the AMF.

S611. The AMF determines, based on the information about the targetaccess network, that the terminal is to be handed over from the firstnetwork to the second network.

S612. The AMF sends a bearer context request to the SMF+PGW-C, where thebearer context request is used to request the bearer contextcorresponding to the PDN connection. Correspondingly, the SMF+PGW-Creceives the bearer context request from the AMF.

S613. The SMF+PGW-C sends the bearer context corresponding to the PDNconnection to the AMF based on the bearer context request.Correspondingly, the AMF receives the bearer context corresponding tothe PDN connection from the SMF+PGW-C.

S614. The AMF sends the bearer context corresponding to the PDNconnection to the MME in the second network. Correspondingly, the MMEreceives the bearer context corresponding to the PDN connection from theAMF.

S615. The MME obtains the tunnel information of the PDN connection fromthe bearer context corresponding to the PDN connection.

S616. The MME sends the tunnel information of the PDN connection to theSGW in the second network. Correspondingly, the SGW receives the tunnelinformation of the PDN connection from the MME.

It should be further understood that, the user plane tunnel and thecontrol plane tunnel in the session establishment/modification phase andthe handover preparation phase are an uplink user plane tunnel on thePGW-U+UPF and an uplink control plane tunnel on the PGW-C+SMF.

It should be noted that, after S610 to S616, an uplink tunnel from theSGW to the UPF+PGW-U has been established, and when the terminal ishanded over from the source access network in the first network to thetarget access network in the second network, uplink data can beseamlessly switched to the uplink tunnel from the SGW to the UPF+PGW-Uand be sent to a data network.

It should be understood that, the handover preparation phase in themethod 600 is not limited to only the foregoing process, and may furtherinclude another process (for example, reference may be made to a processin a handover preparation phase in other approaches). This embodiment ofthis application is not limited thereto.

3. Handover Execution Phase

S617. When the terminal accesses the second network using the targetaccess network, the target access network sends a handover notificationto the MME, where the handover notification is used to notify that theterminal has been handed over from the first network to the secondnetwork. Correspondingly, the MME receives the handover notification.

S618. The MME sends a bearer modification request to the SGW, where thebearer modification request carries downlink tunnel information of theaccess network, and the downlink tunnel information of the accessnetwork includes downlink user plane tunnel information corresponding toeach of the at least one bearer on the access network. Correspondingly,the SGW receives the bearer modification request from the MME.

S619. The SGW sends, based on the bearer modification request in S618, abearer modification request to the SMF+PGW-C through the control planetunnel corresponding to the PDN connection, where the bearermodification request carries downlink tunnel information of the SGW, andthe downlink tunnel information of the SGW includes downlink user planetunnel information corresponding to each of the at least one bearer onthe SGW. Correspondingly, the SMF+PGW-C receives the bearer modificationrequest from the SGW through the control plane tunnel corresponding tothe PDN connection.

S620. The SMF+PGW-C determines, based on a message sent by the SGW, thatthe terminal has been handed over to the second network.

S621. The SMF+PGW-C sends an N4 session establishment/modificationrequest to the UPF+PGW-U, where the N4 sessionestablishment/modification request carries downlink tunnel informationon the SGW and a downlink data transmission rule, and the downlink datatransmission rule is used to indicate the user plane function networkelement corresponding to the PDU session to forward received downlinkdata to the SGW through a corresponding user plane tunnel.Correspondingly, the UPF+PGW-U receives the N4 sessionestablishment/modification request from the SMF+PGW-C.

S622. The UPF+PGW-U sends an N4 session establishment/modificationresponse to the SMF+PGW-C based on the N4 sessionestablishment/modification request received in S621. Correspondingly,the SMF+PGW-C receives the N4 session establishment/modificationresponse from the UPF+PGW-U.

Optionally, the third session management request may be carried in asecond N4 session modification request that is sent by the S1VIF+PGW-Cto the UPF+PGW-U through the N4 interface, and the third sessionmanagement response may be carried in a second N4 session modificationresponse that is sent by the UPF+PGW-U to the S1VIF+PGW-C through the N4interface. This is not limited in this embodiment of this application.

In a possible implementation, the N4 session establishment/modificationrequest in S621 may not carry the downlink data transmission rule, andthe downlink data transmission rule may be carried in the N4 sessionestablishment/modification request in S604 a or S603 b, and the N4session establishment/modification request in S604 a or S603 b mayfurther carry first rule indication information, where the first ruleindication information is used to indicate not to enable the downlinkdata transmission rule.

Correspondingly, the N4 session establishment/modification request inS621 may carry second rule indication information, where the second ruleindication information is used to indicate to enable the downlink datapacket transmission rule. Additionally, the UPF+PGW-U enables thedownlink data packet transmission rule according to the second ruleindication information.

Based on the foregoing description, after S622, a downlink tunnel fromthe UPF+PGW-U to the SGW has been established, and after the terminal ishanded over to the second network, downlink data in the data network maybe switched to the user plane tunnel from the UPF+PGW-U to the SGW andbe sent to the target access network.

FIG. 7A, FIG. 7B, and FIG. 7C are a schematic flowchart of a handovermethod 700 according to an embodiment of this application. The method700 may be applied to the system architecture in the roaming scenarioshown in FIG. 2, or the method 700 may be applied to another similararchitecture.

It should be understood that, the method 700 includes three phases intotal: a session establishment/modification phase, a handoverpreparation phase, and a handover execution phase. The followingdescribes the three phases in detail with reference to FIG. 7A, FIG. 7B,and FIG. 7C.

1. Session Establishment/Modification Phase

It should be noted that, S701 and S702 are a process in which theSMF+PGW-C determines at least one bearer that needs to be establishedwhen a PDU session of a terminal in a first network is to be switched toa PDN connection in a second network.

S701 and S702 are similar to S601 and S602.

It should be noted that, S703 a to S705 a or S703 b to S705 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a user planetunnel corresponding to each of the at least one bearer.

S703 a to S705 a are similar to S603 a to S605 a.

S703 b to S705 b are similar to S603 b to S605 b.

After S703 a or S705 b, the SMF-PGW-C stores user plane tunnelinformation corresponding to each of the at least one bearer into abearer context corresponding to the PDN connection.

It should be noted that, S706 is a process in which the SMF-PGW-Cestablishes a control plane tunnel corresponding to the PDN connectionfor the PDN connection.

S706 is similar to S606.

After S706, the SMF-PGW-C stores control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

It should be understood that, the control plane tunnel corresponding tothe PDN connection is used to transmit control signaling of the PDNconnection to the UPF+PGW-U. In other words, a granularity of thecontrol plane tunnel corresponding to the PDN connection is per PDNconnection basis. Therefore, S706 is performed only in a sessionestablishment phase, that is, S706 is not performed in a sessionmodification phase.

It should be noted that, S707 to S709 are a process in which theSMF+PGW-C obtains a bearer identifier corresponding to each of the atleast one bearer.

S707. The SMF+PGW-C sends a bearer update request to the AMF using thev-SMF, where the bearer update request is used to request the beareridentifier corresponding to each of the at least one bearer.Correspondingly, the AMF receives the bearer update request from theSMF+PGW-C using the v-SMF.

It should be understood that, in S707, the v-SMF provides only a messagetransferring function, that is, forwards, to the AMF, a message that issent by the SMF+PGW-C to the AMF.

For example, the v-SMF may send, to a corresponding AMF based on anidentifier of the terminal, a message that is sent by the SMF+PGW-C tothe AMF, and send, to a corresponding SMF+PGW-C based on an identifierof the PDU session and the identifier of the terminal, a message that issent by the AMF to the SMF+PGW-C.

S708 is similar to S608.

S709. The AMF sends a bearer update response to the SMF+PGW-C using thev-SMF, where the bearer update response carries the bearer identifiercorresponding to each of the at least one bearer. Correspondingly, theSMF+PGW-C receives the bearer update response from the AMF using thev-SMF.

It should be understood that, in S709, the v-SMF provides only a messagetransferring function, that is, forwards, to the SMF+PGW-C, a messagethat is sent by the AMF to the SMF+PGW-C.

After S709, the SMF+PGW-C stores the bearer identifier of each of the atleast one bearer into the bearer context corresponding to the PDNconnection.

S710. The SMF+PGW-C sends, to the v-SMF, the bearer contextcorresponding to the PDN connection, where the bearer contextcorresponding to the PDN connection includes the control plane tunnelinformation corresponding to the PDN connection, the user plane tunnelinformation corresponding to each of the at least one bearer, and thebearer identifier corresponding to each of the at least one bearer.

After S710, the v-SMF stores the bearer context corresponding to the PDNconnection.

Optionally, the control plane tunnel corresponding to the PDN connectionis used to transmit control signaling of the PDN connection to theUPF+PGW-U. In other words, a granularity of the control plane tunnelcorresponding to the PDN connection is per PDN connection basis.Therefore, the control plane tunnel corresponding to the PDN connectionis included only in the bearer context corresponding to the PDNconnection in S710 in the session establishment phase. In other words,the control plane tunnel corresponding to the PDN connection is notincluded in the bearer context corresponding to the PDN connection thatis sent by the SMF+PGW-C to the v-SMF in S710 in the sessionmodification phase. Actually, S710 in the session modification phase isan update of the bearer context corresponding to the PDN connection, tobe more specific, the bearer context corresponding to the PDN connectionincludes a bearer identifier of a newly added bearer and correspondinguser plane tunnel information.

Optionally, there is no sequence among the process of 703 a to 705 a (or703 b to 705 b), the process of S706, and the process of S707 to S709.This is not limited in this embodiment of this application.

It should be noted that, if the process of 703 a to 705 a (or 703 b to705 b) is performed prior to the process of S707 to S709, after theSMF+PGW-C receives a bearer identifier response in S709, if allocationof a bearer identifier requested by a specific bearer fails, theSMF+PGW-C sends a message to the PGW-U+UPF, to delete a user planetunnel corresponding to the bearer.

Based on the foregoing description, after S701 to S710, the bearercontext corresponding to the PDN connection stored by the v-SMF and theSMF-PGW-C may include the control plane tunnel information correspondingto the PDN connection, the user plane tunnel information correspondingto each of the at least one bearer, and the bearer identifiercorresponding to each of the at least one bearer.

Optionally, the SMF+PGW-C may further send, to the v-SMF, otherinformation that is needed in the subsequent handover preparation phaseand handover execution phase, for example, a QoS parameter of eachbearer, where the QoS parameter of each bearer includes at least one ofa QoS class identifier (QCI), an allocation retention priority (ARP), apacket filter, or a packet filter priority. Correspondingly, the v-SMFreceives the information from the SMF+PGW-C, and stores the informationinto the bearer context corresponding to the PDN connection. This is notlimited in this embodiment of this application.

2. Handover Preparation Phase

S711 and S712 are similar to S610 and S611.

S713. The AMF sends a bearer context request to the v-SMF, where thebearer context request is used to request the bearer contextcorresponding to the PDN connection. Correspondingly, the v-SMF receivesthe bearer context request from the AMF.

S714. The v-SMF sends the bearer context corresponding to the PDNconnection to the AMF based on the bearer context request.Correspondingly, the AMF receives the bearer context corresponding tothe PDN connection from the v-SMF.

S715 to S717 are similar to S614 to S616.

It should be understood that, the handover preparation phase in themethod 700 is not limited to only the foregoing process, and may furtherinclude another process (for example, reference may be made to a processin a handover preparation phase in other approaches). This embodiment ofthis application is not limited thereto.

3. Handover execution phase: This phase is the same as the handoverexecution phase in the method 600.

It should be noted that, the v-SMF in the method 700 has a function oftransparently transmitting a message between the SMF+PGW-C and the AMF.

FIG. 8A and FIG. 8B are a schematic flowchart of a handover method 800according to an embodiment of this application. The method 800 may beapplied to the system architecture in the roaming scenario shown in FIG.2, or the method 800 may be applied to another similar architecture.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. This is notlimited in this embodiment of this application.

It should be further understood that, the method 800 includes threephases in total: a session establishment/modification phase, a handoverpreparation phase, and a handover execution phase. The followingdescribes the three phases in detail with reference to FIG. 8A and FIG.8B.

1. Session Establishment/Modification Phase

It should be noted that, S801 and S802 are a process in which theSMF+PGW-C determines at least one bearer that needs to be establishedwhen a PDU session of a terminal in a first network is to be switched toa PDN connection in a second network.

S801 and S802 are similar to S601 and S602.

It should be noted that, S803 a to S805 a or S803 b to S805 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a user planetunnel corresponding to each of the at least one bearer.

S803 a to S805 a are similar to S603 a to S605 a.

S803 b to 8805 b are similar to S603 b to S605 b.

After S803 a or S805 b, the SMF-PGW-C stores user plane tunnelinformation corresponding to each of the at least one bearer and anuplink data transmission rule into a bearer context corresponding to thePDN connection.

It should be noted that, S806 is a process in which the SMF-PGW-Cestablishes a control plane tunnel corresponding to the PDN connectionfor the PDN connection.

S806 is similar to S606.

After S806, the SMF-PGW-C stores control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

It should be understood that, the control plane tunnel corresponding tothe PDN connection is used to transmit control signaling of the PDNconnection to the UPF+PGW-U. In other words, a granularity of thecontrol plane tunnel corresponding to the PDN connection is per PDNconnection basis. Therefore, S806 is performed only in a sessionestablishment phase, that is, S806 is not performed in a sessionmodification phase.

It should be noted that, S807 to S811 are a process in which theSMF+PGW-C obtains a bearer identifier corresponding to each of the atleast one bearer.

S807. The SMF+PGW-C sends tunnel information of the PDN connection tothe v-SMF, where the tunnel information of the PDN connection includesthe control plane tunnel information corresponding to the PDN connectionand the user plane tunnel information corresponding to each of the atleast one bearer. Correspondingly, the v-SMF receives the tunnelinformation of the PDN connection from the SMF+PGW-C.

Optionally, the control plane tunnel corresponding to the PDN connectionis used to transmit control signaling of the PDN connection to theUPF+PGW-U. In other words, a granularity of the control plane tunnelcorresponding to the PDN connection is per PDN connection basis.Therefore, the control plane tunnel corresponding to the PDN connectionis included in the bearer context corresponding to the PDN connection inS807 in the session establishment phase. In other words, the controlplane tunnel corresponding to the PDN connection is not included in thetunnel information of the PDN connection sent by the SMF+PGW-C to thev-SMF in S807 in the session modification phase. Actually, S807 in thesession modification phase is an update of the bearer contextcorresponding to the PDN connection, to be more specific, the tunnelinformation corresponding to the PDN connection includes a beareridentifier of a newly added bearer and corresponding user plane tunnelinformation.

After S807, the v-SMF stores the user plane tunnel informationcorresponding to each bearer and the control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

S808. The v-SMF sends a bearer identifier request to the AMF based onthe tunnel information of the PDN connection received in S807, where thebearer identifier request is used to request to allocate a beareridentifier to each of the at least one bearer. Correspondingly, the AMFreceives the bearer identifier request from the v-SMF.

S809. The AMF allocates the bearer identifier to each of the at leastone bearer based on the bearer identifier request.

S810. The AMF sends bearer identifier information to the v-SMF, wherethe bearer identifier information includes the bearer identifiercorresponding to each of the at least one bearer. Correspondingly, thev-SMF receives the bearer identifier information from the AMF.

After S810, the v-SMF stores the bearer identifier of each bearer intothe bearer context corresponding to the PDN connection.

It should be noted that, S811 is a process in which the SMF+PGW-Cobtains the bearer identifier of each bearer.

S811. The v-SMF sends the bearer identifier information to theSMF-PGW-C. Correspondingly, the SMF+PGW-C receives the bearer identifierinformation from the v-SMF.

For example, the v-SMF sends both the bearer identifier corresponding toeach bearer and the user plane tunnel information corresponding to eachbearer to the SMF+PGW-C, such that the SMF+PGW-C associates the beareridentifier corresponding to each bearer with the user plane tunnelinformation.

After S811, the SMF-PGW-C stores the bearer identifier corresponding toeach of the at least one bearer into the bearer context corresponding tothe PDN connection.

Optionally, there is no sequence between the process of S803 a to S805 a(or S803 b to S805 b) and the process of S806. This is not limited inthis embodiment of this application.

Based on the foregoing description, after S801 to S811, the bearercontext corresponding to the PDN connection stored by the v-SMF and theSMF-PGW-C may include the control plane tunnel information correspondingto the PDN connection, the user plane tunnel information correspondingto each bearer, and the bearer identifier of each bearer. The bearercontext corresponding to the PDN connection is used to hand over theterminal from the first network to the second network.

Optionally, the SMF+PGW-C may further send, to the v-SMF, otherinformation that is needed in the subsequent handover preparation phaseand handover execution phase, for example, a QoS parameter of eachbearer, where the QoS parameter of each bearer includes at least one ofa QoS class identifier (QCI), an allocation retention priority (ARP), apacket filter, or a packet filter priority. Correspondingly, the v-SMFreceives the information from the SMF+PGW-C, and stores the informationinto the bearer context corresponding to the PDN connection. This is notlimited in this embodiment of this application.

2. Handover preparation phase: This phase is the same as the handoverpreparation phase in the method 700.

3. Handover execution phase: This phase is the same as the handoverexecution phase in FIG. 6A, FIG. 6B, and FIG. 6C.

It should be noted that, the v-SMF in the method 800 has a processingfunction.

For example, the v-SMF may check whether a bearer identifier associatedwith the user plane tunnel information corresponding to each bearerexists in the tunnel information of the PDN connection. If no beareridentifier is allocated to the at least one bearer, the v-SMF requeststhe AMF to allocate the bearer identifier to each of the at least onebearer.

The foregoing describes, with reference to FIG. 6A to FIG. 8B, thehandover methods provided in the embodiments of this application, wherethe SMF-PGW-C or the v-SMF obtains the tunnel information of the PDNconnection in the session establishment/modification phase. Thefollowing describes, with reference to FIG. 9A to FIG. 11B, a case inwhich the SMF-PGW-C or the v-SMF obtains the tunnel information of thePDN connection in the handover preparation phase.

FIG. 9A, FIG. 9B, and FIG. 9C are schematic flowcharts of a handovermethod 900 according to an embodiment of this application. The method900 may be applied to the system architecture in the non-roamingscenario shown in FIG. 1, or the method 900 may be applied to anothersimilar architecture.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. This is notlimited in this embodiment of this application.

It should be further understood that, the method 900 includes threephases in total: a session establishment/modification phase, a handoverpreparation phase, and a handover execution phase. The followingdescribes the three phases in detail.

1. Session Modification Phase

It should be noted that, S901 and S902 are a process in which theSMF+PGW-C determines at least one bearer that needs to be establishedwhen a PDU session of a terminal in a first network is to be switched toa PDN connection in a second network.

S901 and S902 are similar to S601 and S602.

It should be noted that, S903 to S905 are a process in which theSMF+PGW-C obtains a bearer identifier corresponding to each of the atleast one bearer.

S903 to S905 are similar to S607 to S609.

After S905, the SMF-PGW-C stores a bearer identifier corresponding toeach bearer into a bearer context corresponding to the PDN connection.

Based on the foregoing description, after S905, the bearer contextcorresponding to the PDN connection stored by the SMF+PGW-C may includethe bearer identifier corresponding to each bearer.

2. Handover Preparation Phase

It should be understood that, S906 to S908 are similar to S610 to S612.Details are not described herein again.

It should be noted that, S909 a to S911 a or S909 b to S911 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a user planetunnel corresponding to each bearer.

It should be understood that, S909 a to S911 a are similar to S603 a toS605 a, and S909 b to S911 b are similar to S603 b to S605 b. Detailsare not described herein again.

After S909 a or S911 b, the SMF-PGW-C stores user plane tunnelinformation corresponding to each bearer into the bearer contextcorresponding to the PDN connection.

It should be noted that, S912 is a process in which the SMF-PGW-Cestablishes a control plane tunnel corresponding to the PDN connectionfor the PDN connection.

It should be understood that, S912 is the same as S606.

After S912, the SMF-PGW-C stores control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

It should be understood that, subsequent S913 to S916 are similar toS613 to S616. Details are not described herein again.

Based on the foregoing description, after S916, an uplink tunnel fromthe SGW to the UPF+PGW-U has been established, and when the terminal ishanded over from a source access network in the first network to atarget access network in the second network, uplink data can beseamlessly switched to the uplink tunnel from the SGW to the UPF+PGW-Uand be sent to a data network.

3. Handover execution phase: This phase is the same as the handoverexecution phase in FIG. 6A, FIG. 6B, and FIG. 6C.

FIG. 10A, FIG. 10B, and FIG. 10C are schematic flowcharts of a handovermethod 1000 according to an embodiment of this application. The method1000 may be applied to the system architecture in the roaming scenarioshown in FIG. 2, or the method 1000 may be applied to another similararchitecture.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. This is notlimited in this embodiment of this application.

It should be further understood that, the method 1000 includes threephases in total: a session establishment/modification phase, a handoverpreparation phase, and a handover execution phase. The followingdescribes the three phases in detail.

1. Session Establishment/Modification Phase

It should be noted that, S1001 and S1002 are a process in which theSMF+PGW-C determines at least one bearer that needs to be establishedwhen a PDU session of a terminal in a first network is to be switched toa PDN connection in a second network.

S1001 and S1002 are similar to S601 and S602.

It should be noted that, S1003 to S1005 are a process in which thePGW-C+SMF obtains a bearer identifier corresponding to each bearer.

S1003 to S1005 are similar to S707 to S709.

After S1005, the SMF-PGW-C stores the bearer identifier corresponding toeach bearer into a bearer context corresponding to the PDN connection.

2. Handover Preparation Phase

It should be understood that, S1006 and S1007 are similar to S610 andS611. Details are not described herein again.

S1008. The AMF sends a bearer context request to the SMF+PGW-C using thev-SMF, where the bearer context request is used to request the bearercontext corresponding to the PDN connection. Correspondingly, theSMF+PGW-C receives the bearer context request from the AMF using thev-SMF.

It should be understood that, in S1008, the v-SMF provides only amessage transferring function, that is, forwards, to the SMF+PGW-C, amessage that is sent by the AMF to the SMF+PGW-C.

It should be noted that, S1009 a to S1011 a or S1009 b to S1011 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a user planetunnel corresponding to each bearer.

It should be understood that, S1009 a to S101 la are similar to S603 ato S605 a, and S1009 b to S1011 b are similar to S603 b to S605 b.Details are not described herein again.

After S1009 a or S10010 b, the SMF-PGW-C stores user plane tunnelinformation corresponding to each bearer into the bearer contextcorresponding to the PDN connection.

It should be noted that, S1012 is a process in which the SMF-PGW-Cestablishes a control plane tunnel corresponding to the PDN connectionfor the PDN connection.

After S1012, the SMF-PGW-C stores control plane tunnel informationcorresponding to the PDN connection into the bearer contextcorresponding to the PDN connection.

S1013. The SMF+PGW-C sends the bearer context corresponding to the PDNconnection to the AMF using the v-SMF. Correspondingly, the AMF receivesthe bearer context corresponding to the PDN connection from theSMF+PGW-C. It should be understood that, in S1013, the v-SMF providesonly a message transferring function, that is, forwards, to the AMF, amessage that is sent by the SMF+PGW-C to the AMF.

It should be understood that, subsequent S1014 to S1016 are similar toS614 to S616. Details are not described herein again.

Based on the foregoing description, after S1016, an uplink tunnel fromthe SGW to the UPF+PGW-U has been established, and when the terminal ishanded over from a source access network in the first network to atarget access network in the second network, uplink data can beseamlessly switched to the uplink tunnel from the SGW to the UPF+PGW-Uand be sent to a data network.

3. Handover execution phase: This phase is the same as the handoverexecution phase in FIG. 6A, FIG. 6B, and FIG. 6C.

The foregoing describes, with reference to FIG. 6A to FIG. 10C, aprocess in which the terminal is handed over from the 5G network to the4G network, and the following describes, with reference to FIG. 11A,FIG. 11B, FIG. 12A, FIG. 12B, and FIG. 12C, a process in which theterminal is handed over from the 4G network to the 5G network.

FIG. 11A and FIG. 11B are schematic flowcharts of a handover method 1100according to an embodiment of this application. The method 1100 may beapplied to the system architecture in the non-roaming scenario shown inFIG. 1, or the method 1100 may be applied to another similararchitecture.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. This is notlimited in this embodiment of this application.

It should be further understood that, there is no control plane tunnelcorresponding to a PDU session in the first network. Therefore, in thisembodiment of this application, a tunnel corresponding to the PDUsession is a user plane tunnel corresponding to the PDU session, andtunnel information corresponding to the PDU session is user plane tunnelinformation corresponding to the PDU session.

S1101. The SMF+PGW-C receives a session establishment/modificationrequest from the AMF, where the session establishment/modificationrequest is used to request to establish a PDN connection in the secondnetwork or trigger modification of the PDN connection in the secondnetwork. Correspondingly, the AMF sends the sessionestablishment/modification request to the SMF+PGW-C.

S1102. The SMF+PGW-C obtains, based on the first request message, tunnelinformation of the PDU session when the PDN connection of the terminalin the second network is to be switched to the PDU session in the firstnetwork.

It should be noted that, S1103 a to S1105 a or S1103 b to S1105 b are aprocess in which the SMF+PGW-C and the UPF+PGW-U establish a tunnel forthe PDU session.

Optionally, tunnel information corresponding to the PDU session may beallocated by the SMF+PGW-C or the UPF+PGW-U. This is not limited in thisembodiment of this application.

If the SMF+PGW-C allocates the tunnel information, S1103 a to S1105 aare performed. If the UPF+PGW-U allocates the tunnel information, S1103b to S1105 b are performed.

(1) The SMF+PGW-C allocates the tunnel information to the PDU session.

S1103 a. The SMF+PGW-C allocates user plane tunnel information to thePDU session.

S1104 a. The SMF+PGW-C sends an N4 session establishment/modificationrequest to the UPF+PGW-U, where the N4 sessionestablishment/modification request is used to request to establish thetunnel corresponding to the PDU session, the N4 sessionestablishment/modification request carries the tunnel information of thePDU session and an uplink data transmission rule, and the uplink datatransmission rule is used to indicate a user plane function networkelement corresponding to the PDN connection how to forward uplink datathat is received through the user plane tunnel corresponding to the PDUsession. Correspondingly, the UPF+PGW-U receives the N4 sessionestablishment/modification request from the SMF+PGW-C.

S1105 a. The UPF+PGW-U sends an N4 session establishment/modificationresponse to the SMF+PGW-C based on the N4 sessionestablishment/modification request received in S1104 a. Correspondingly,the SMF+PGW-C receives the N4 session establishment/modificationresponse from the UPF+PGW-U.

(2) The UPF+PGW-U allocates the tunnel information to the PDU session.

S1103 b. The SMF+PGW-C sends an N4 session establishment/modificationrequest to the UPF+PGW-U, where the N4 sessionestablishment/modification request is used to request to establish thetunnel corresponding to the PDU session, the N4 sessionestablishment/modification request carries the tunnel information of thePDU session and an uplink data transmission rule, and the uplink datatransmission rule is used to indicate a user plane function networkelement corresponding to the PDN connection how to forward uplink datathat is received through the user plane tunnel corresponding to the PDUsession. Correspondingly, the UPF+PGW-U receives the N4 sessionestablishment/modification request from the SMF-PGW-C.

S1104 b. The UPF+PGW-U allocates the tunnel information to the PDUsession based on the N4 session establishment/modification requestreceived in S1103 b.

S1105 b. The UPF+PGW-U sends an N4 session establishment/modificationresponse to the SMF+PGW-C, where the N4 sessionestablishment/modification response carries the tunnel informationcorresponding to the PDU session. Correspondingly, the SMF+PGW-Creceives the N4 session establishment/modification response from theUPF+PGW-U.

After S1105 b, the SMF+PGW-C stores the tunnel information correspondingto the PDU session.

Optionally, the SMF+PGW-C or the UPF+PGW-U may use a tunnelcorresponding to one bearer in the second network as the tunnelcorresponding to the PDU session in the first network, for example, usea tunnel of a default bearer as the tunnel corresponding to the PDUsession. In this case, uplink tunnel information allocated by theSMF+PGW-C to the PDU session is tunnel information of a user planetunnel corresponding to the default bearer on the PGW-U+UPF.Alternatively, the SMF+PGW-C may allocate new uplink tunnel informationto the PDU session in the first network.

This embodiment of this application provides the following possibleimplementations:

In an optional manner 1, only S1103 a is performed, and S1104 a andS1105 a are not performed.

In an optional manner 2, S1103 a to S1105 a are performed.

In an optional manner 3, S1103 b to S1105 b are performed.

In an optional manner 4, neither S1103 a to 1105 a nor S1103 b to 1105 bare performed.

For example, the SMF+PGW-C determines the uplink data transmission rulebased on traffic filter template (TFT) information of each bearer of thePDN connection, where a TFT includes a packet filter of a data flowsupported by a bearer.

S1106. A source access network (such as an eNB in FIG. 11A and FIG. 11B)in the second network sends a handover request to the MME in the secondnetwork, where the handover request carries information about a targetaccess network (such as a gNB in the first network). Correspondingly,the MME receives the handover request from the source access network.

It should be understood that, the access network in this embodiment ofthis application may be, for example, a base station.

S1107. The MME selects an AMF based on the information about the targetaccess network, and sends a handover request to the AMF, where thehandover request carries an EPS bearer context. Correspondingly, the AMFreceives the handover request from the MME.

S1108. The AMF sends a session establishment request to an S1VIF+PGW-Ccorresponding to each PDN connection in the EPS bearer context, wherethe session establishment request carries a default bearer identifiercorresponding to the PDN connection. Correspondingly, the SMF+PGW-Creceives the session establishment request from the AMF.

In a possible implementation, the default bearer identifier carried inthe session establishment request in S1108 may be used to indicate theSMF+PGW-C to prepare for handing over the terminal from the secondnetwork to the first network.

In another possible implementation, the session establishment request inS1108 may further carry first indication information, where the firstindication information is used to indicate the SMF+PGW-C to prepare forhanding over the terminal from the second network to the first network,and the first indication information may be, for example, at least onebit.

If the manner 1 is used, S1104 a and S1105 a are performed after S1108.

If the manner 4 is used, the manner 2 or the manner 3 is performed afterS1108.

S1109. The SMF+PGW-C queries a session context based on the defaultbearer identifier, and generates an N2 session request based oninformation in the session context, where the N2 session requestincludes an identifier of the PDU session, an S-NSSAI corresponding tothe PDU session, a session AMBR, and information about each QoS flow inthe PDU session.

Optionally, the N2 session request message further includes the uplinktunnel information of the PDU session on the UPF+PGW-U.

S1110. The SMF+PGW-C sends the N2 session request to the AMF.Correspondingly, the AMF receives the N2 session request from theSMF+PGW-C.

Optionally, after S1110, the AMF may indicate the target access networkto prepare for handing over the terminal from the second network to thefirst network, for example, indicate the target access network toallocate a radio resource, and allocate tunnel information of the accessnetwork (such as a downlink tunnel of the access network) to ato-be-switched session.

S1111. After the terminal accesses the target access network, the AMFsends a session update request to the SMF+PGW-C, where the sessionupdate request carries received downlink tunnel informationcorresponding to the access network. Correspondingly, the SMF+PGW-Creceives the session update request from the AMF.

Optionally, the session update request carries third indicationinformation, where the third indication information is used to indicatethe SMF+PGW-C to switch a path to the first network, for example, thePGW-U+UPF is switched to send downlink data using the access network inthe first network.

S1112. The SMF+PGW-C sends an N4 session update request to theUPF+PGW-U, where the N4 session update request carries a downlinkforwarding rule and the downlink tunnel information of the target accessnetwork, where the downlink data transmission rule is used to indicateto send, through the user plane tunnel corresponding to the PDU session,a downlink data packet received by the user plane function networkelement corresponding to the PDN connection. Correspondingly, theUPF+PGW-U receives the N4 session update request from the SMF+PGW-C.

S1113. The UPF+PGW-U sends an N4 session update response to theSMF+PGW-C.

In a possible implementation, the N4 session update request in S1112 maynot carry the downlink data transmission rule, the downlink datatransmission rule may be carried in the N4 sessionestablishment/modification request in S1104 a or S1103 b, and the N4session establishment/modification request in S1104 a or S1103 b furthercarries first rule indication information, where the first ruleindication information is used to indicate not to enable the downlinkdata transmission rule.

Correspondingly, the N4 session update request in S1112 may carry secondrule indication information, where the second rule indicationinformation is used to indicate to enable the downlink data packettransmission rule. Additionally, the UPF+PGW-U enables the downlink datapacket transmission rule according to the second rule indicationinformation.

FIG. 12A, FIG. 12B, and FIG. 12C are schematic flowcharts of a handovermethod 1200 according to an embodiment of this application. The method1200 may be applied to the system architecture in the roaming scenarioshown in FIG. 2, or the method 1200 may be applied to another similararchitecture.

It should be understood that, the first network may include a 5Gnetwork, and the second network may include a 4G network. This is notlimited in this embodiment of this application.

It should be further understood that, there is no control plane tunnelcorresponding to a PDU session in the first network. Therefore, in thisembodiment of this application, a tunnel corresponding to the PDUsession is a user plane tunnel corresponding to the PDU session, andtunnel information corresponding to the PDU session is user plane tunnelinformation corresponding to the PDU session.

S1201 and S1202 are similar to S1101 and S1102.

S1203 a to S1205 a are similar to S1103 a to S1105 a.

S1203 b to S1205 b are similar to S1103 b to S1105 b.

This embodiment of this application provides the following possibleimplementations:

In an optional manner 1, only S1203 a is performed, and S1204 a andS1205 a are not performed.

In an optional manner 2, S1203 a to S1205 a are performed.

In an optional manner 3, S1203 b to S1205 b are performed.

In an optional manner 4, neither S1203 a to 1205 a nor S1203 b to 1205 bare performed.

S1206. A source access network (such as an eNB in FIG. 12A, FIG. 12B,and FIG. 12C) in the second network sends a handover request to the MMEin the second network, where the handover request carries informationabout a target access network (such as a gNB in the first network).Correspondingly, the MME receives the handover request from the sourceaccess network.

It should be understood that, the access network in this embodiment ofthis application may be, for example, a base station.

S1207 is similar to S1107.

S1208. The AMF selects a v-SMF, and sends a session establishmentrequest to the v-SMF, where the session establishment request carries adefault bearer identifier corresponding to the PDN connection and anidentifier of an SMF+PGW-C corresponding to the PDN connection in an EPSbearer context. Correspondingly, the v-SMF receives the sessionestablishment request from the AMF.

Optionally, the session establishment request carries fourth indicationinformation, where the fourth indication information is used to indicateto prepare for handing over the terminal from the second network to thefirst network.

Optionally, the fourth indication information may be the default beareridentifier.

Optionally, the fourth session management request carries the defaultbearer identifier and the fourth indication information, where thefourth indication information is used to indicate to prepare for handingover the terminal from the second network to the first network, and thefourth indication information may be at least one bit.

S1209. The v-SMF sends a session modification request to the SMF+PGW-Cbased on the identifier of the SMF+PGW-C, where the session modificationrequest carries the default bearer identifier corresponding to the PDNconnection. Correspondingly, the SMF+PGW-C receives the sessionmodification request from the v-SMF.

In a possible implementation, the default bearer identifier carried inthe session modification request in S1209 may be used to indicate theSMF+PGW-C to prepare for handing over the terminal from the secondnetwork to the first network.

In another possible implementation, the session modification request inS1209 may further carry first indication information, where the firstindication information is used to indicate the SMF+PGW-C to prepare forhanding over the terminal from the second network to the first network,and the first indication information may be, for example, at least onebit.

If the manner 1 is used, S1204 a and S1205 a are performed after S1209.

If the manner 4 is used, the manner 2 or the manner 3 is performed afterS1209.

S1210. The SMF+PGW-C queries a session context based on the defaultbearer identifier, and generates an N2 session request based oninformation in the session context, where the N2 session requestincludes an identifier of the PDU session, an S-NSSAI corresponding tothe PDU session, a session AMBR, and information about each QoS flow inthe PDU session.

S1211. The SMF+PGW-C sends, to the v-SMF, the N2 session requestreceived in S1210 and uplink tunnel information of the PDU session onthe UPF+PGW-U. Correspondingly, the v-SMF receives, from the SMF+PGW-C,the N2 session request received in S1210 and the uplink tunnelinformation of the PDU session on the UPF+PGW-U.

S1212. The v-SMF allocates a v-UPF, and sends an N4 session request tothe v-UPF, where the N4 session request carries the uplink tunnelinformation of the PDU session on the UPF+PGW-U. Correspondingly, thev-UPF receives the N4 session request from the v-SMF.

Optionally, after S1212, a corresponding uplink tunnel from the v-UPF tothe UPF+PGW-U is established on the v-UPF for the PDU session, and N3uplink tunnel information on the v-UPF is allocated.

S1213. The v-SMF adds the N3 uplink tunnel information on the v-UPF tothe N2 session request received in S1211.

S1214. The v-SMF sends a session establishment response to the AMF, andsends an N2 session request to the AMF. Correspondingly, the AMFreceives the session establishment response and the N2 session requestfrom the v-SMF.

Optionally, after S1214, the AMF may indicate the target base station toprepare for handing over the terminal from the second network to thefirst network, for example, indicate the target base station to allocatea radio resource, and allocate tunnel information of the base station(such as a downlink tunnel of the base station) to a to-be-switchedsession.

S1215. After UE accesses the target access network (such as the gNB inFIG. 12A, FIG. 12B, and FIG. 12C), the target access network sends ahandover notification message to the AMF. Correspondingly, the AMFreceives the handover notification message from the target accessnetwork.

S1216. The AMF sends a session update request to the v-SMF, where thesession update request carries received downlink tunnel information ofthe access network. Correspondingly, the v-SMF receives the sessionupdate request from the AMF.

Optionally, the message carries handover complete indicationinformation, and the v-SMF may determine, based on the indication, thathandover is completed.

S1217. The v-SMF sends an N4 session update request to the v-UPF, wherethe N4 session update request carries the downlink tunnel information ofthe target access network. Correspondingly, the v-UPF receives the N4session update request from the v-SMF.

S1218. The v-SMF sends a session update request to the SMF+PGW-C, wherethe session update request carries downlink tunnel information of thev-UPF. Correspondingly, the SMF+PGW-C receives the session updaterequest from the v-SMF.

Optionally, the session update request further carries handover completeindication information, where the handover complete indicationinformation is used to indicate that handover has been completed.

S1219. The SMF+PGW-C sends an N4 session update request to theUPF+PGW-U, where the N4 session update request carries a downlink datatransmission rule and the downlink tunnel information of the v-UPF, thedownlink data transmission rule is used to indicate to send, through theuser plane tunnel corresponding to the PDU session, a downlink datapacket received by a user plane function network element correspondingto the PDN connection.

S1220. The SMF+PGW-C sends a session update response to the v-SMF.Correspondingly, the v-SMF receives the session update response from theSMF+PGW-C.

S1221. The v-SMF sends the session update response to the AMF.Correspondingly, the AMF receives the session update response from thev-SMF.

In a possible implementation, the N4 session update request in S1219 maynot carry the downlink data transmission rule, the downlink datatransmission rule may be carried in the N4 sessionestablishment/modification request in S1204 a or S1203 b, and the N4session establishment/modification request further carries firstindication information, where the first indication information is usedto indicate not to enable the downlink data transmission rule.

Correspondingly, the third session management request in S1219 may carrysecond indication information, where the second indication informationis used to indicate to enable the downlink data packet transmissionrule. Additionally, the UPF+PGW-U enables the downlink data packettransmission rule according to the second indication information.

Optionally, the SMF+PGW-C may determine, based on the handover completeindication information in S1218, that handover is completed, in order toindicate the PGW-U+UPF to enable downlink forwarding, or send thedownlink data transmission rule to the PGW-U+UPF, in order to enabledownlink forwarding.

The foregoing describes, with reference to FIG. 3 to FIG. 12C, thehandover methods provided in the embodiments of this application, andthe following describes, with reference to FIG. 13 to FIG. 18, handoverapparatuses provided in embodiments of this application.

FIG. 13 is a schematic block diagram of a handover apparatus 1300according to an embodiment of this application. The handover apparatus1300 includes a determining unit 1310 and an obtaining unit 1320.

The determining unit 1310 is configured to determine at least one bearerthat needs to be established when a protocol data unit (PDU) session ofa terminal in a first network is to be switched to a packet data network(PDN) connection in a second network.

The obtaining unit 1320 is configured to obtain tunnel information ofthe PDN connection, where the tunnel information of the PDN connectionincludes user plane tunnel information corresponding to each of the atleast one bearer.

Optionally, the obtaining unit 1320 is configured to: allocate thecorresponding user plane tunnel information to each of the at least onebearer; or request a user plane function network element correspondingto the PDU session to allocate the user plane tunnel informationcorresponding to each bearer; and obtain the user plane tunnelinformation corresponding to each bearer from the user plane functionnetwork element corresponding to the PDU session.

Optionally, the tunnel information of the PDN connection furtherincludes control plane tunnel information corresponding to the PDNconnection, and the obtaining unit 1320 is further configured to obtainthe control plane tunnel information corresponding to the PDNconnection.

Optionally, the apparatus 1300 further includes a first sending unitconfigured to send a first session management request to the user planefunction network element corresponding to the PDU session, where thefirst session management request is used to request to establish ormodify a user plane tunnel corresponding to each of the at least onebearer, the first session management request carries an uplink datatransmission rule, and the uplink data transmission rule is used toindicate the user plane function network element corresponding to thePDU session how to forward uplink data that is received through the userplane tunnel corresponding to the PDN connection.

Optionally, the obtaining unit 1320 is further configured to obtain abearer identifier corresponding to each of the at least one bearer.

Optionally, the obtaining unit 1320 is configured to: send a beareridentifier request to a mobility management network element using asecond control plane function network element, where the beareridentifier request is used to request to allocate a bearer identifier toeach of the at least one bearer, the second control plane functionnetwork element is a visited control plane network element correspondingto the PDU session, and the apparatus is a home control plane networkelement corresponding to the PDU session; and receive the beareridentifier corresponding to each of the at least one bearer from themobility management network element using the second control planefunction network element.

Optionally, the apparatus 1300 further includes a second sending unitconfigured to send the tunnel information of the PDN connection and thebearer identifier corresponding to each of the at least one bearer tothe second control plane function network element.

Optionally, the apparatus 1300 further includes a third sending unitconfigured to send the tunnel information of the PDN connection to asecond control plane function network element, where the second controlplane function network element is a visited control plane networkelement corresponding to the PDU session, and the apparatus is a homecontrol plane network element corresponding to the PDU session.Additionally, the obtaining unit 1320 is further configured to receivethe bearer identifier corresponding to each of the at least one bearerfrom the second control plane function network element.

Optionally, the obtaining unit 1320 is configured to obtain the tunnelinformation of the PDN connection when determining that the PDU sessionof the terminal needs to be established in the first network or the PDUsession in the first network needs to be modified.

Optionally, the apparatus 1300 further includes a fourth sending unitconfigured to send, to the user plane function network elementcorresponding to the PDU session, a downlink data transmission rulecorresponding to the PDN connection, where the downlink datatransmission rule is used to indicate the user plane function networkelement corresponding to the PDU session to forward received downlinkdata through a corresponding user plane tunnel.

It should be noted that in this embodiment of this application, thedetermining unit 1310 may be implemented using a processor, and theobtaining unit 1320 may be implemented using a transceiver. As shown inFIG. 14, a handover apparatus 1400 may include a processor 1410, amemory 1420, and a transceiver 1430. The memory 1420 may be configuredto store code executed by the processor 1410 and the like, and theprocessor 1410 may be configured to process data or a program.

In an implementation process, steps of the foregoing methods can beimplemented using an integrated logic circuit of hardware in theprocessor 1410, or using instructions in a form of software. The stepsof the methods disclosed with reference to the embodiments of thisapplication may be directly performed by a hardware processor, or may beperformed using a combination of hardware in the processor and asoftware module. The software module may be located in a mature storagemedium in the art, such as a random access memory, a flash memory, aread-only memory, a programmable read-only memory, an electricallyerasable programmable memory, or a register. The storage medium islocated in the memory 1420, and the processor 1410 reads information inthe memory 1420 and completes the steps of the foregoing methods incombination with hardware of the processor. To avoid repetition, detailsare not described herein again.

The handover apparatus 1300 shown in FIG. 13 or the handover apparatus1400 shown in FIG. 14 can implement processes corresponding to the firstcontrol plane function network element in the foregoing methodembodiments. For the handover apparatus 1300 or the handover apparatus1400, reference may be made to the foregoing description. To avoidrepetition, details are not described herein again.

FIG. 15 is a schematic block diagram of another handover apparatus 1500according to an embodiment of this application. The handover apparatusincludes an obtaining unit 1510 and a sending unit 1520.

The obtaining unit 1510 is configured to receive tunnel information of aPDN connection from a first control plane function network element,where the tunnel information of the PDN connection includes user planetunnel information corresponding to each of at least one bearer, theapparatus is a visited control plane network element corresponding tothe PDN connection, and the first control plane function network elementis a home control plane network element corresponding to the PDNconnection.

The sending unit 1520 is configured to send the tunnel information ofthe PDN connection to a mobility management network element.

Optionally, the tunnel information of the PDN connection furtherincludes control plane tunnel information corresponding to the PDNconnection.

Optionally, the sending unit 1520 is further configured to: forward, tothe mobility management network element, a bearer identifier requestthat is sent by the first control plane function network element to themobility management network element, where the bearer identifier requestis used to request to allocate a bearer identifier to each of the atleast one bearer; and forward, to the first control plane functionnetwork element, the bearer identifier that corresponds to each of theat least one bearer and that is sent by the mobility management networkelement to the first control plane function network element.

Optionally, the obtaining unit 1510 is further configured to receive thebearer identifier corresponding to each of the at least one bearer fromthe first control plane function network element.

Optionally, the sending unit 1520 is further configured to send a beareridentifier request to the mobility management network element based onthe tunnel information of the PDN connection, where the beareridentifier request is used to request to allocate a bearer identifier toeach of the at least one bearer. Additionally, the obtaining unit 1510is further configured to receive the bearer identifier corresponding toeach of the at least one bearer from the mobility management networkelement.

Optionally, the apparatus further includes a storage unit, where thestorage unit is configured to store the bearer identifier correspondingto each of the at least one bearer. The sending unit 1520 is furtherconfigured to send the bearer identifier corresponding to each of the atleast one bearer to the first control plane function network element.

It should be noted that in this embodiment of this application, theobtaining unit 1510 and the sending unit 1520 may be implemented using atransceiver. As shown in FIG. 16, a handover apparatus 1600 may includea processor 1610, a memory 1620, and a transceiver 1630. The memory 1620may be configured to store code executed by the processor 1610 and thelike, and the processor 1610 may be configured to process data or aprogram.

In an implementation process, steps of the foregoing methods can beimplemented using an integrated logic circuit of hardware in theprocessor 1610, or using instructions in a form of software. The stepsof the methods disclosed with reference to the embodiments of thisapplication may be directly performed by a hardware processor, or may beperformed using a combination of hardware in the processor and asoftware module. The software module may be located in a mature storagemedium in the art, such as a random access memory, a flash memory, aread-only memory, a programmable read-only memory, an electricallyerasable programmable memory, or a register. The storage medium islocated in the memory 1620, and the processor 1610 reads information inthe memory 1620 and completes the steps of the foregoing methods incombination with hardware of the processor. To avoid repetition, detailsare not described herein again.

The handover apparatus 1500 shown in FIG. 15 or the handover apparatus1600 shown in FIG. 16 can implement processes corresponding to thesecond control plane network element in the foregoing methodembodiments. For the handover apparatus 1500 or the handover apparatus1600, reference may be made to the foregoing description. To avoidrepetition, details are not described herein again.

FIG. 17 is a schematic block diagram of a handover apparatus 1700according to an embodiment of this application. The handover apparatus1700 includes a determining unit 1710 and an obtaining unit 1720.

The determining unit 1710 is configured to determine that a PDNconnection of a terminal in a second network needs to be switched to aPDU session in a first network.

The obtaining unit 1720 is configured to obtain user plane tunnelinformation corresponding to the PDU session.

Optionally, the obtaining unit 1720 is configured to: allocate, to thePDU session, the user plane tunnel information corresponding to the PDUsession; or request a user plane function network element correspondingto the PDN connection to allocate user plane tunnel informationcorresponding to the PDU session, and obtain the user plane tunnelinformation that corresponds to the PDU session from the user planefunction network element corresponding to the PDN connection.

Optionally, the apparatus 1700 further includes a first sending unitconfigured to send a first session management request to the user planefunction network element corresponding to the PDN connection, where thefirst session management request is used to request to establish ormodify a user plane tunnel corresponding to the PDU session, the firstsession management request carries an uplink data transmission rule, andthe uplink data transmission rule is used to indicate the user planefunction network element corresponding to the PDN connection how toforward uplink data that is received through the user plane tunnelcorresponding to the PDU session.

Optionally, the first sending unit is further configured such that whendetermining that the uplink data transmission rule changes, the firstsending unit sends an updated uplink data transmission rule to the userplane function network element corresponding to the PDN connection.

Optionally, the obtaining unit 1720 is further configured to receive asecond session management request, where the second session managementrequest carries first indication information, and the first indicationinformation is used to indicate to prepare for handing over the terminalfrom the second network to the first network.

Optionally, the apparatus 1700 further includes a second sending unitconfigured to send, to the user plane function network elementcorresponding to the PDN connection, a downlink data transmission rulecorresponding to the PDU session, where the downlink data transmissionrule is used to indicate to send, through the user plane tunnelcorresponding to the PDU session, a downlink data packet received by theuser plane function network element corresponding to the PDN connection.

It should be noted that in this embodiment of this application, thedetermining unit 1710 may be implemented using a processor, and theobtaining unit 1720 may be implemented using a transceiver. As shown inFIG. 18, a communications apparatus 1800 may include a processor 1810, amemory 1820, and a transceiver 1830. The memory 1820 may be configuredto store code executed by the processor 1810 and the like, and theprocessor 1810 may be configured to process data or a program.

In an implementation process, steps of the foregoing methods can beimplemented using an integrated logic circuit of hardware in theprocessor 1810, or using instructions in a form of software. The stepsof the methods disclosed with reference to the embodiments of thisapplication may be directly performed by a hardware processor, or may beperformed using a combination of hardware in the processor and asoftware module. The software module may be located in a mature storagemedium in the art, such as a random access memory, a flash memory, aread-only memory, a programmable read-only memory, an electricallyerasable programmable memory, or a register. The storage medium islocated in the memory 1820, and the processor 1810 reads information inthe memory 1820 and completes the steps of the foregoing methods incombination with hardware of the processor. To avoid repetition, detailsare not described herein again.

The handover apparatus 1700 shown in FIG. 17 or the handover apparatus1800 shown in FIG. 18 can implement processes corresponding to the firstcontrol plane function network element in the foregoing methodembodiments. For the handover apparatus 1700 or the handover apparatus1800, reference may be made to the foregoing description. To avoidrepetition, details are not described herein again.

An embodiment of this application further provides a computer readablemedium, configured to store a computer program. The computer programincludes an instruction used to perform the method corresponding to thefirst control plane function network element or the second control planefunction network element in the foregoing method embodiments.

An embodiment of this application further provides a computer programproduct. The computer program product includes computer program code.When the computer program code is executed by a communications unit anda processing unit or a transceiver and a processor of a communicationsdevice (such as a terminal device or a network device), thecommunications device is enabled to perform the method corresponding tothe first control plane function network element or the second controlplane function network element in any one of the foregoing methodembodiments.

An embodiment of this application further provides a communicationschip, where the communications chip stores an instruction. When theinstruction runs on a communications apparatus, the communications chipis enabled to perform the method corresponding to the first controlplane function network element or the second control plane functionnetwork element in the foregoing method embodiments.

The embodiments in this application may be separately or jointly used.This is not limited herein.

It should be understood that, descriptions such as “first” and “second”in the embodiments of this application are only used as examples andused to distinguish between objects, but do not indicate a sequence orindicate a specific limitation on a quantity of devices in theembodiments of this application, and cannot constitute any limitation onthe embodiments of this application.

It should be understood that sequence numbers of the foregoing processesdo not mean execution sequences in various embodiments of thisapplication. The execution sequences of the processes should bedetermined based on functions and internal logic of the processes, andshould not be construed as any limitation on the implementationprocesses of the embodiments of this application.

A person of ordinary skill in the art may be aware that, units andalgorithm steps in the examples described with reference to theembodiments disclosed in this specification may be implemented byelectronic hardware or a combination of computer software and electronichardware. Whether the functions are performed by hardware or softwaredepends on particular applications and design constraints of thetechnical solutions. A person skilled in the art may use differentmethods to implement the described functions for each particularapplication, but it should not be considered that the implementationgoes beyond the scope of this application.

It may be clearly understood by a person skilled in the art that, forthe purpose of ease and brevity of description, for detailed workingprocesses of the foregoing system, apparatus, and unit, reference may bemade to corresponding processes in the foregoing method embodiments, anddetails are not described herein again.

In the several embodiments provided in this application, it should beunderstood that the disclosed system, apparatus, and method may beimplemented in other manners. For example, the described apparatusembodiment is merely an example. For example, the unit division ismerely logical function division and may be other division in actualimplementation. For example, a plurality of units or components may becombined or may be integrated into another system, or some features maybe ignored or not performed. In addition, the displayed or discussedmutual couplings or direct couplings or communication connections may beimplemented using some interfaces. The indirect couplings orcommunication connections between the apparatuses or units may beimplemented in electrical, mechanical, or other forms.

The units described as separate parts may or may not be physicallyseparate, and parts displayed as units may or may not be physical units,may be located in one position, or may be distributed on a plurality ofnetwork units. Some or all of the units may be selected depending onactual requirements, to achieve the objectives of the solutions of theembodiments.

In addition, functional units in the embodiments of this application maybe integrated into one processing unit, or each of the units may existalone physically, or two or more units may be integrated into one unit.

When the functions are implemented in a form of a software functionalunit and sold or used as an independent product, the functions may bestored in a computer-readable storage medium. Based on such anunderstanding, the technical solutions of this application essentially,or the part contributing to other approaches, or some of the technicalsolutions may be implemented in a form of a software product. Thecomputer software product is stored in a storage medium, and includesseveral instructions for instructing a computer device (which may be apersonal computer, a server, a network device, or the like) to performall or some of the steps of the methods described in the embodiments ofthis application. The foregoing storage medium includes any medium thatcan store program code, such as a Universal Serial Bus (USB) flashdrive, a removable hard disk, a read-only memory (ROM), a random accessmemory (RAM), a magnetic disk, or an optical disc.

The foregoing descriptions are merely example implementations of thisapplication, but are not intended to limit the protection scope of thisapplication. Any variation or replacement readily figured out by aperson skilled in the art within the technical scope disclosed in thisapplication shall fall within the protection scope of this application.Therefore, the protection scope of this application shall be subject tothe protection scope of the claims.

1. A control plane function network element, comprising: a memoryconfigured to store program instructions; and a processor coupled to thememory and configured to execute the program instructions, such thatduring an establishment or a modification of a protocol data unit (PDU)session of a terminal in a first network, the program instructions causethe control plane function network element to: send a bearer identifierrequest to a mobility management network element to request to allocatea bearer identifier for the PDU session; receive a bearer identifierallocated to each bearer of at least one bearer from the mobilitymanagement network element, wherein the at least one bearer is includedin a packet data network (PDN) connection in a second network thatcorresponds to the PDU session in the first network; and obtain tunnelinformation of the PDN connection, wherein the tunnel informationcomprises control plane tunnel information corresponding to the PDNconnection and user plane tunnel information corresponding to eachbearer of the at least one bearer.
 2. The control plane function networkelement of claim 1, wherein the processor is configured to execute theprogram instructions to further cause the control plane function networkelement to determine that the PDU session supports interworking of thefirst network and the second network.
 3. The control plane functionnetwork element of claim 1, wherein the processor is configured toexecute the program instructions to further cause the control planefunction network element to allocate the user plane tunnel informationto each bearer of the at least one bearer.
 4. The control plane functionnetwork element of claim 1, wherein the processor is configured toexecute the program instructions to further cause the control planefunction network element to: request a user plane function networkelement corresponding to the PDU session to allocate the user planetunnel information to each bearer of the at least one bearer; and obtainthe user plane tunnel information from the user plane function networkelement.
 5. The control plane function network element of claim 1,wherein the processor is configured to execute the program instructionsto further cause the control plane function network element to: send,via a second control plane function network element, the beareridentifier request to the mobility management network element; andreceive, via the second control plane function network element, thebearer identifier, wherein the control plane function network element isa home control plane network element of the terminal, and wherein thesecond control plane function network element is a visited control planenetwork element of the terminal.
 6. The control plane function networkelement of claim 1, wherein the processor is configured to execute theprogram instructions to further cause the control plane function networkelement to store the tunnel information and the bearer identifier into abearer context corresponding to the PDN connection.
 7. The control planefunction network element of claim 1, wherein the processor is configuredto execute the program instructions to further cause the control planefunction network element to send the tunnel information and the beareridentifier to a second control plane function network element, whereinthe control plane function network element is a home control planenetwork element of the terminal, and wherein the second control planefunction network element is a visited control plane network element ofthe terminal.
 8. The control plane function network element of claim 1,wherein the processor is configured to execute the program instructionsto further cause the control plane function network element to send asession management request to a user plane function network elementcorresponding to the PDU session, wherein the session management requestcomprises an uplink data transmission rule, and wherein the uplink datatransmission rule indicates to the user plane function network elementhow to forward uplink data received through a user plane tunnelcorresponding to each bearer of the at least one bearer.
 9. A secondcontrol plane function network element, comprising: a memory configuredto store program instructions; and a processor coupled to the memory andconfigured to execute the program instructions to cause the secondcontrol plane function network element to: receive, from a first controlplane function network element during an establishment or a modificationof a protocol data unit (PDU) session of a terminal in a first network,a bearer context corresponding to a packet data network (PDN) connectionin a second network, wherein the PDN connection corresponds to the PDUsession and comprises at least one bearer, and wherein the bearercontext comprises user plane tunnel information corresponding to eachbearer of the at least one bearer, control plane tunnel informationcorresponding to the PDN connection, and a bearer identifiercorresponding to each bearer of the at least one bearer; store thebearer context; receive a bearer context request from an access andmobility management function network element in the first network; andsend the bearer context to the access and mobility management functionnetwork element in response to the bearer context request.
 10. Thesecond control plane function network element of claim 9, wherein thesecond control plane function network element is a visited control planenetwork element of the terminal, and wherein the first control planefunction network element is a home control plane network element of theterminal.
 11. A method in a communications system, the methodcomprising: during an establishment or a modification of a protocol dataunit (PDU) session of a terminal in a first network: sending, by a firstcontrol plane function network element, a bearer identifier request to amobility management network element to request to allocate a beareridentifier for the PDU session; receiving, by the first control planefunction network element, a bearer identifier allocated to each bearerof at least one bearer from the mobility management network element,wherein the at least one bearer is included in a packet data network(PDN) connection in a second network that corresponds to the PDU sessionin the first network; and obtaining, by the first control plane functionnetwork element, tunnel information of the PDN connection, wherein thetunnel information comprises control plane tunnel informationcorresponding to the PDN connection and user plane tunnel informationcorresponding to each bearer of the at least one bearer.
 12. The methodof claim 11, further comprising determining, by the first control planefunction network element, that the PDU session supports interworking ofthe first network and the second network.
 13. The method of claim 11,further comprising allocating, by the first control plane functionnetwork element, the user plane tunnel information to each bearer of theat least one bearer.
 14. The method of claim 11, further comprising:requesting, by the first control plane function network element, a userplane function network element corresponding to the PDU session toallocate the user plane tunnel information to each bearer of the atleast one bearer; and receiving, by the first control plane functionnetwork element, the user plane tunnel information from the user planefunction network element.
 15. The method of claim 11, furthercomprising: sending, by the first control plane function network elementvia a second control plane function network element, the beareridentifier request to the mobility management network element; andreceiving, by the first control plane function network element via thesecond control plane function network element, the bearer identifier,wherein the control plane function network element is a home controlplane network element of the terminal, and wherein the second controlplane function network element is a visited control plane networkelement of the terminal.
 16. The method of claim 11, further comprisingstoring, by the first control plane function network element, the tunnelinformation and the bearer identifier into a bearer contextcorresponding to the PDN connection.
 17. The method of claim 11, furthercomprising sending, by the first control plane function network element,the tunnel information and the bearer identifier to a second controlplane function network element, wherein the first control plane functionnetwork element is a home control plane network element of the terminal,and wherein the second control plane function network element is avisited control plane network element of the terminal.
 18. The method ofclaim 11, further comprising sending, by the first control planefunction network element, a session management request to a user planefunction network element corresponding to the PDU session, wherein thesession management request comprises an uplink data transmission rule,and wherein the uplink data transmission rule indicates to the userplane function network element how to forward uplink data that isreceived through a user plane tunnel corresponding to each bearer of theat least one bearer.
 19. A method in a communications system, the methodcomprising: receiving, by a second control plane function networkelement during an establishment or a modification of a protocol dataunit (PDU) session of a terminal in a first network, a bearer contextcorresponding to a packet data network (PDN) connection in a secondnetwork, wherein the bearer context is from a first control planefunction network element, wherein the PDN connection corresponds to thePDU session and comprises at least one bearer, and wherein the bearercontext comprises user plane tunnel information corresponding to eachbearer of the at least one bearer, control plane tunnel informationcorresponding to the PDN connection, and a bearer identifiercorresponding to each of the at least one bearer; storing, by the secondcontrol plane function network element, the bearer context; receiving,by the second control plane function network element, a bearer contextrequest from an access and mobility management function network elementin the first network; and sending, by the second control plane functionnetwork element, the bearer context to the access and mobilitymanagement function network element in response to the bearer contextrequest.
 20. The method of claim 19, wherein the second control planefunction network element is a visited control plane network element ofthe terminal, and wherein the first control plane function networkelement is a home control plane network element of the terminal.